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+.. SPDX-License-Identifier: GPL-2.0
+
+=====================
+X86 PVM Specification
+=====================
+
+Underlying states
+-----------------
+
+**The PVM guest is only running on the underlying CPU with underlying
+CPL=3.**
+
+The term ``underlying`` refers to the actual hardware architecture
+state. For example ``underlying CR3`` is the physic ``CR3`` of the
+architecture. On the contrary, ``CR3`` or ``PVM CR3`` is the virtualized
+``PVM CR3`` register. Any x86 states or registers in this document are
+PVM states or registers unless any of "underlying", "physic", or
+"hardware" is used to describe the states or registers. The doc uses
+"underlying" mostly to describe the actual hardware architecture.
+
+When the PVM guest is only running on the underlying CPU, it not only
+runs with underlying CPL=3 but also with the following underlying states
+and registers:
+
++-------------------+--------------------------------------------------+
+| Registers         | Values                                           |
++===================+==================================================+
+| Underlying RFLAGS | IOPL=VM=VIF=VIP=0, IF=1, fixed-bit1=1.           |
++-------------------+--------------------------------------------------+
+| Underlying CR3    | implementation-defined value, typically          |
+|                   | shadows the ``PVM CR3`` with extra pages         |
+|                   | mapped including the switcher.                   |
++-------------------+--------------------------------------------------+
+| Underlying CR0    | PE=PG=WP=ET=NE=AM=MP=1, CD=NW=EM=TS=0            |
++-------------------+--------------------------------------------------+
+| Underlying CR4    | VME=PVI=0, PAE=FSGSBASE=1,                       |
+|                   | others=implementation-defined                    |
++-------------------+--------------------------------------------------+
+| Underlying EFER   | SCE=LMA=LME=1, NXE=implementation-defined.       |
++-------------------+--------------------------------------------------+
+| Underlying GDTR   | All Entries with DPL<3 in the table are          |
+|                   | hypervisor-defined values. The table must        |
+|                   | have entries with DPL=3 for the selectors:       |
+|                   | ``__USER32_CS``, ``__USER_CS``,                  |
+|                   | ``__USER_DS`` (``__USER32_CS`` is                |
+|                   | implementation-defined value,                    |
+|                   | ``__USER_CS``\ =\ ``__USER32_CS``\ +8,           |
+|                   | ``__USER_DS``\ =\ ``__USER32_CS``\ +16)          |
+|                   | and may have other hypervisor-defined            |
+|                   | DPL=3 data entries. Typically a                  |
+|                   | host-defined CPUNODE entry is also in the        |
+|                   | underlying ``GDT`` table for each host CPU       |
+|                   | and its content (segment limit) can be           |
+|                   | visible to the PVM guest.                        |
++-------------------+--------------------------------------------------+
+| Underlying TR     | implementation-defined, no IOPORT is             |
+|                   | allowed.                                         |
++-------------------+--------------------------------------------------+
+| Underlying LDTR   | must be NULL                                     |
++-------------------+--------------------------------------------------+
+| Underlying IDT    | implementation-defined. All gate entries         |
+|                   | are with DPL=0, except for the entries for       |
+|                   | vector=3,4 and a vector>32 (for legacy           |
+|                   | syscall, normally 0x80) with DPL=3.              |
++-------------------+--------------------------------------------------+
+| Underlying CS     | loaded with the selector ``__USER_CS`` or        |
+|                   | ``__USER32_CS``.                                 |
++-------------------+--------------------------------------------------+
+| Underlying SS     | loaded with the selector ``__USER_DS``.          |
++-------------------+--------------------------------------------------+
+| Underlying        | loaded with the selector NULL or                 |
+| DS/ES/FS/GS       | ``__USER_DS`` or other DPL=3 data entries        |
+|                   | in the underlying ``GDT`` table.                 |
++-------------------+--------------------------------------------------+
+| Underlying DR6    | 0xFFFF0FF0, until a hardware #DB is              |
+|                   | delivered and the hardware exits the guest       |
++-------------------+--------------------------------------------------+
+| Underlying DR7    | ``DR7_GD``\ =0; illegitimate linear              |
+|                   | address (see address space separation) in        |
+|                   | ``DR0-DR3`` causes the corresponding bits        |
+|                   | in the ``underlying DR7`` cleared.               |
++-------------------+--------------------------------------------------+
+
+In summary, the underlying states are typical x86 states to run
+unprivileged software with stricter limitations.
+
+PVM modes and states
+--------------------
+
+PVM has three PVM modes and they are modified IA32-e mode with PVM ABI.
+
+- PVM 64bit supervisor mode: modified X86 64bit supervisor mode with
+  PVM ABI
+
+- PVM 64bit user mode: X86 64bit user mode with PVM event handling
+
+- PVM 32bit compatible user mode: x86 compatible user mode with PVM
+  event handling
+
+| A VMM or hypervisor may also support non-PVM mode. They are non-IA32-e
+  mode or IA32-e compatible kernel mode.
+| The specification has nothing to do with any non-PVM mode. But if the
+  hypervisor or the VMM can not boot the software directly into PVM
+  mode, the hypervisor can implement non-PVM mode as bootstrap.
+| Bootstrapping is implementation-defined. Bootstrapping in non-PVM mode
+  should conform to pure X86 ABI until it enters X86 64bit supervisor
+  mode and then the PVM hypervisor changes privilege registers(``CR0``,
+  ``CR4,`` ``EFER``, ``MSR_STAR``) to conform to PVM mode and transits
+  it into PVM 64bit supervisor mode.
+
+States or registers on PVM modes
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
++-----------------------+----------------------------------------------+
+| Register              | Values                                       |
++=======================+==============================================+
+| ``CR3`` and           | PVM switches ``CR3`` with                    |
+| MSR_PVM_SWITCH_CR3    | MSR_PVM_SWITCH_CR3 when switching            |
+|                       | supervisor/user mode. Hypercall              |
+|                       | HC_LOAD_PGTBL can load ``CR3`` and           |
+|                       | MSR_PVM_SWITCH_CR3 in one call. It           |
+|                       | is recommended software to use               |
+|                       | different ``CR3`` for supervisor             |
+|                       | and user modes like KPTI.                    |
++-----------------------+----------------------------------------------+
+| ``CR0``               | PE=PG=WP=ET=NE=AM=MP=1,                      |
+|                       | CD=NW=EM=TS=0                                |
++-----------------------+----------------------------------------------+
+| ``CR4``               | VME/PVI=0; PAE/FSGSBASE=1;                   |
+|                       | UMIP/PKE/OSXSAVE/OSXMMEXCPT/OSFXSR=host.     |
+|                       | PCID is recommended to be set even           |
+|                       | if the ``underlying CR4.PCID`` is            |
+|                       | not set. SMAP=SMEP=0 and the                 |
+|                       | corresponding features are                   |
+|                       | disabled in CPUID leaves.                    |
++-----------------------+----------------------------------------------+
+| ``EFER``              | SCE=LMA=LME=1; NXE=underlying;               |
++-----------------------+----------------------------------------------+
+| ``RFLAGS``            | Mapped to the underlying RFLAGS except for   |
+|                       | the RFLAGS.IF. (The underlying RFLAGS.IF     |
+|                       | is always 1.)                                |
+|                       |                                              |
+|                       | IOPL=VM=VIF=VIP=0, fixed-bit1=1.             |
+|                       | AC is not recommended to be set in           |
+|                       | the supervisor mode.                         |
+|                       |                                              |
+|                       | The PVM interrupt flag is defined as:        |
+|                       |                                              |
+|                       | - the bit 9 of the PVCS::event_flags when in |
+|                       |   supervisor mode.                           |
+|                       | - 1, when in user mode.                      |
+|                       | - 0, when in supervisor mode if              |
+|                       |   MSR_PVM_VCPU_CTRL_STRUCT=0.                |
++-----------------------+----------------------------------------------+
+| ``GDTR``              | ignored (can be written and read             |
+|                       | to get the last written value but            |
+|                       | take no effect). The effective PVM           |
+|                       | ``GDT`` table can be considered to           |
+|                       | be a read-only table consisting of           |
+|                       | entries: emulated supervisor mode            |
+|                       | ``CS/SS`` and entries in                     |
+|                       | ``underlying GDT`` with DPL=3. The           |
+|                       | hypercall PVM_HC_LOAD_TLS can                |
+|                       | modify part of the                           |
+|                       | ``underlying GDT``.                          |
++-----------------------+----------------------------------------------+
+| ``TR``                | ignored. Replaced by PVM event               |
+|                       | handling                                     |
++-----------------------+----------------------------------------------+
+| ``IDT``               | ignored. Replaced by PVM event               |
+|                       | handling                                     |
++-----------------------+----------------------------------------------+
+| ``LDTR``              | ignored. No replacement so it can            |
+|                       | be considered disabled.                      |
++-----------------------+----------------------------------------------+
+| ``CS`` in             | emulated. the ``underlying CS`` is           |
+| supervisor mode       | ``__USER_CS``.                               |
++-----------------------+----------------------------------------------+
+| ``CS`` in             | mapped to the ``underlying CS``              |
+| user mode             | which is ``__USER_CS`` or                    |
+|                       | ``__USER32_CS``                              |
++-----------------------+----------------------------------------------+
+| ``SS`` in             | emulated. the ``underlying SS`` is           |
+| supervisor mode       | ``__USER_DS``.                               |
++-----------------------+----------------------------------------------+
+| ``SS`` in             | mapped to the ``underlying SS``              |
+| user mode             | whose value is ``__USER_DS``.                |
++-----------------------+----------------------------------------------+
+| DS/ES/FS/GS           | mapped to the                                |
+|                       | ``underlying DS/ES/FS/GS``, loaded           |
+|                       | with the selector NULL or                    |
+|                       | ``__USER_DS`` or other DPL=3 data            |
+|                       | entries in the ``underlying GDT``            |
+|                       | table.                                       |
++-----------------------+----------------------------------------------+
+| interrupt shadow mask | no interrupt shadow mask                     |
++-----------------------+----------------------------------------------+
+| NMI shadow mask       | set when #NMI is delivered and               |
+|                       | cleared when and only when                   |
+|                       | EVENT_RETURN_USER or                         |
+|                       | EVENT_RETURN_SUPERVISOR                      |
++-----------------------+----------------------------------------------+
+
+MSR_PVM_VCPU_CTRL_STRUCT
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. code::
+
+   struct pvm_vcpu_struct {
+       u64 event_flags;
+       u32 event_errcode;
+       u32 event_vector;
+       u64 cr2;
+       u64 reserved0[5];
+
+       u16 user_cs, user_ss;
+       u32 reserved1;
+       u64 reserved2;
+       u64 user_gsbase;
+       u32 eflags;
+       u32 pkru;
+       u64 rip;
+       u64 rsp;
+       u64 rcx;
+       u64 r11;
+   }
+
+PVCS::event_flags
+^^^^^^^^^^^^^^^^^
+
+| ``PVCS::event_flags.IF``\ (bit 9): interrupt enable flag: The flag
+  is set to respond to maskable external interrupts; and cleared to
+  inhibit maskable external interrupts.
+|   The flag works only in supervisor mode. The VCPU always responds to
+    maskable external interrupts regardless of the value of this flag in
+    user mode. The flag is unchanged when the VCPU switches
+    user/supervisor modes, even when handling the synthetic instruction
+    EVENT_RETURN_USER. The guest is responsible for clearing the flag
+    before switching to user mode (issuing EVENT_RETURN_USER) to ensure
+    that the external interrupt is disabled when the VCPU is switched back
+    from user mode later.
+
+| ``PVCS::event_flags.IP``\ (bit 8): interrupt pending flag: The
+  hypervisor sets it if it fails to inject a maskable event to the VCPU
+  due to the interrupt-enable flag being cleared in supervisor mode.
+|   The guest is responsible for issuing a hypercall PVM_HC_IRQ_WIN when
+    the guest sees this bit after setting the PVCS::event_flags.IF.
+    The hypervisor clears this bit in handling
+    PVM_HC_IRQ_WIN/IRQ_HLT/EVENT_RETURN_USER/EVENT_RETURN_HYPERVISOR.
+
+Other bits are reserved (Software should set them to zero).
+
+PVCS::event_vector, PVCS::event_errcode
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+If the vector event being delivered is from user mode or with vector >= 32
+from supervisor mode ``PVCS::event_vector`` is set to the vector number. And
+if the event has an error code, ``PVCS::event_errcode`` is set to the code.
+
+PVCS::cr2
+^^^^^^^^^
+
+If the event being delivered is a page fault (#PF), ``PVCS::cr2`` is set
+to be ``CR2`` (the faulting linear address).
+
+PVCS::user_cs, PVCS::user_ss, PVCS::user_gsbase, PVCS::pkru, PVCS::rsp, PVCS::eflags, PVCS::rip, PVCS::rcx, PVCS::r11
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+| ``CS``, ``SS``, ``GSBASE``, ``PKRU``, ``RSP``, ``EFLAGS``, ``RIP``,
+  ``RCX``, and ``R11`` are saved to ``PVCS::user_cs``,
+  ``PVCS::user_ss``, ``PVCS::user_gsbase``, ``PVCS::pkru``,
+  ``PVCS::rsp``, ``PVCS::eflags``, ``PVCS::rip``, ``PVCS::rcx``,
+  ``PVCS::r11`` correspondingly when handling the synthetic instruction
+  EVENT_RETURN_USER or vice vers when the architecture is switching to
+  supervisor mode on any event in user mode.
+| The value of ``PVCS::user_gsbase`` is semi-canonicalized before being
+  set to the ``underlying GSBASE`` by adjusting bits 63:N to get the
+  value of bit N–1, where N is the host’s linear address width (48 or
+  57).
+| The value of ``PVCS::eflags`` is standardized before setting to the
+  ``underlying RFLAGS``. IOPL, VM, VIF, and VIP are cleared, and IF and
+  FIXED1 are set.
+| If an event with vector>=32 happens in supervisor mode, ``RSP``,
+  ``EFLAGS``, ``RIP``, ``RCX``, and ``R11`` are saved to ``PVCS::rsp``,
+  ``PVCS::eflags``, ``PVCS::rip``, ``PVCS::rcx``, ``PVCS::r11``
+  correspondingly.
+
+TSC MSRs
+~~~~~~~~
+
+TSC ABI is not settled down yet.
+
+X86 MSR
+~~~~~~~
+
+MSR_GS_BASE/MSR_KERNEL_GS_BASE
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+``MSR_GS_BASE`` is mapped to the ``underlying GSBASE``.
+
+The ``MSR_KERNEL_GS_BASE`` is recommended to be synced with
+``MSR_GS_BASE`` when in supervisor mode, and supervisor software is
+recommended to maintain its version of ``MSR_KERNEL_GS_BASE``, and
+``PVCS::user_gsbase`` is recommended to be used on this purpose.
+
+When the CPU is switching from user mode to supervisor mode,
+``PVCS::user_gsbase`` is updated as the value of ``MSR_GS_BASE`` (the
+``underlying GSBASE``), and the value of ``MSR_GS_BASE`` is reset to
+``MSR_KERNEL_GS_BASE`` atomically at the same time.
+
+When the CPU is switching from supervisor mode to user mode,
+``MSR_KERNEL_GS_BASE`` is normally set with the value of
+``MSR_GS_BASE`` (but the hypervisor is allowed to omit this operation
+because ``MSR_GS_BASE`` and ``MSR_KERNEL_GS_BASE`` are expected to be
+the same when in supervisor), and the ``MSR_GS_BASE`` is loaded with
+``PVCS::user_gsbase``.
+
+WRGSBASE is not recommended to be used in supervisor mode.
+
+MSR_SYSCALL_MASK
+^^^^^^^^^^^^^^^^
+
+Ignored, when syscall, ``RFLAGS`` is set to a default value.
+
+MSR_STAR
+^^^^^^^^
+
+| ``__USER_CS,`` ``__USER_DS`` derived from it must be the same as
+  host's ``__USER_CS,`` ``__USER_DS`` and have RPL=3. ``__KERNEL_CS``,
+  ``__KERNEL_DS`` derived from it must have RPL=0 and be the same value
+  as the current PVM ``CS`` ``SS`` registers hold respectively.
+  Otherwise #GP.
+| X86 forces RPL for derived ``__USER_CS,`` ``__USER_DS``,
+  ``__USER32_CS``, ``__KERNEL_CS``, (not ``__KERNEL_DS``) when using
+  them, so the RPLs can be an arbitrary value.
+
+MSR_CSTAR, MSR_IA32_SYSENTER_CS/EIP/ESP
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Ignored, the software should use INTn instead for compatibility
+syscalls.
+
+MSR_IA32_PKRS
+^^^^^^^^^^^^^
+
+See "`Protection Keys <#protection-keys>`__".
+
+PVM MSRs
+~~~~~~~~
+
+MSR_PVM_SWITCH_CR3
+^^^^^^^^^^^^^^^^^^
+
+Switched with ``CR3`` when mode switching. No TLB request is issued when
+mode switching.
+
+MSR_PVM_EVENT_ENTRY
+^^^^^^^^^^^^^^^^^^^
+
+| The value is the entry point for vector events from the PVM user mode.
+| The value+256 is the entry point for vector events (vector < 32) from
+  the PVM supervisor mode.
+| The value+512 is the entry point for vector events (vector >= 32) from
+  the PVM supervisor mode.
+
+MSR_PVM_SUPERVISOR_RSP
+^^^^^^^^^^^^^^^^^^^^^^
+
+When switching from supervisor mode to user mode, this MSR is
+automatically saved with ``RSP`` which is restored from it when
+switching back from user mode.
+
+MSR_PVM_SUPERVISOR_REDZONE
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+When delivering the event from supervisor mode, a fixed-size area
+is reserved below the current ``RSP`` and can be safely used by
+guest. The size is specified in this MSR.
+
+MSR_PVM_LINEAR_ADDRESS_RANGE
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+See "`Paging <#paging>`__".
+
+PML4_INDEX_START, PML4_INDEX_END, PML5_INDEX_START, and PML5_INDEX_END
+are encoded in the MSR and they are all 9 bits value with the most
+significant bit set:
+
+- bit 57-63 are all set; bit 48-56: PML5_INDEX_END, bit 56 must be set.
+- bit 41-47 are all set; bit 32-40: PML5_INDEX_START, bit 40 must be set.
+- bit 25-31 are all set; bit 16-24:PML4_INDEX_END, bit 24 must be set.
+- bit 9-15 are all set; bit 0-8:PML4_INDEX_START, bit 8 must be set.
+
+constraints:
+
+- 256 <= PML5_INDEX_START < PML5_INDEX_END < 511
+- 256 <= PML4_INDEX_START < PML4_INDEX_END < 511
+- PML5_INDEX_START = PML5_INDEX_END = 0x1FF if the
+  ``underlying CR4.LA57`` is not set.
+
+The three legitimate address ranges for PVM virtual addresses:
+
+::
+
+  [ (1UL << 48) * (0xFE00 | PML5_INDEX_START), (1UL << 48) * (0xFE00 | PML5_INDEX_END) )
+  [ (1UL << 39) * (0x1FFFE00 | PML4_INDEX_START), (1UL << 39) * (0x1FFFE00 | PML4_INDEX_END) )
+  Lower half address (canonical address with bit63=0)
+
+The MSR is initialized as the widest ranges when the CPU is reset. The
+ranges should be sub-ranges of these initialized ranges when writing to
+the MSR or migration.
+
+| Pagetable walking is confined to these legitimate address ranges.
+| Note:
+
+- the top 2G is not in the range, so the guest supervisor software should
+  be PIE kernel.
+- Breakpoints (``DR0-DR3``) out of these ranges are not activated in the
+  underlying DR7.
+
+MSR_PVM_RETU_RIP, MSR_PVM_RETS_RIP
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The bare SYSCALL instruction staring at ``MSR_PVM_RETU_RIP`` or
+``MSR_PVM_RETS_RIP`` is synthetic instructions to return to
+user/supervisor mode. See "`PVM Synthetic
+Instructions <#pvm-synthetic-instructions>`__" and "`Events and Mode
+Changing <#events-and-mode-changing>`__".
+
+.. pvm-synthetic-instructions:
+
+PVM Synthetic Instructions
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+PVM_SYNTHETIC_CPUID: invlpg 0xffffffffff4d5650;cpuid
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Works the same as the bare CPUID instruction generally, but it is
+ensured to be handled by the PVM hypervisor and reports the corresponding
+CPUID results for PVM.
+
+PVM_SYNTHETIC_CPUID is supposed to not trigger any trap in the real or virtual
+x86 kernel mode and is also guaranteed to trigger a trap in the underlying
+hardware user mode for the hypervisor emulating it. The hypervisor emulates
+both of the basic instructions, while the INVLPG is often emulated as an NOP
+since 0xffffffffff4d5650 is normally out of the allowed linear address ranges.
+
+EVENT_RETURN_SUPERVISOR: SYSCALL instruction starting at MSR_PVM_RETS_RIP
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+EVENT_RETURN_SUPERVISOR instruction returns from supervisor mode to
+supervisor mode with the return state on the stack.
+
+EVENT_RETURN_USER: SYSCALL instruction starting at MSR_PVM_RETU_RIP
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+EVENT_RETURN_USER instruction returns from supervisor mode to user
+mode with the return state on the PVCS.
+
+X86 Instructions with changed behavior
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+CPUID
+^^^^^
+
+Guest CPUID instruction would get the host's CPUID information normally
+(when CPUID faulting is not enabled), and the synthetic instruction
+KVM_CPUID is recommended to be used instead in guest supervisor
+software.
+
+SGDT/SIDT/SLDT/STR/SMSW
+^^^^^^^^^^^^^^^^^^^^^^^
+
+Guest SGDT/SIDT/SLDT/STR/SMSW instructions would get the host's
+information. ``CR4.UMIP`` is in effect for guests only when the host
+enables it.
+
+LAR/LSL/VERR/VERW
+^^^^^^^^^^^^^^^^^
+
+Guest LAR/LSL/VERR/VERW instructions would get segment information from
+host ``GDT``.
+
+STAC/CLAC, SWAPGS, SYSEXIT, SYSRET
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+These instructions are not allowed for PVM supervisor software, using
+them would result in unexpected behavior for the guest.
+
+SYSENTER
+^^^^^^^^
+
+Results in #GP.
+
+INT n
+^^^^^
+
+Only 0x80 and 0x3 are allowed in guests. Other INT n results in #GP.
+
+RDPKRU/WRPKRU
+^^^^^^^^^^^^^
+
+When the guest is in supervisor mode, RDPKRU/WRPKRU would access the
+``underlying PKRU`` register which is effectively PVM's
+``MSR_IA32_PKRS``, so the guest supervisor software should access user
+``PKRU`` via ``PVCS::pkru``.
+
+CPUID leaf
+~~~~~~~~~~
+
+- Features disabled in the host are also disabled in the guest except for
+  some specially handled features such as PCID and PKS.
+
+  - PCID can be enabled even host PCID is disabled or the hardware doesn't
+    support PCID.
+  - PKS can be enabled if the host ``CR4.PKE`` is set because guest PKS is
+    handled via hardware PKE.
+
+- Features that require the hypervisor's handling but are not yet
+  implemented are disabled in the guest.
+
+- Some features that require hardware-privileged instructions are
+  disabled in the guest.
+
+  - XSAVES/XRESTORES/MSR_IA32_XSS is not enabled.
+
+- Features that require distinguishing U/S pages are disabled in the
+  guest.
+
+  - SMEP/SMAP is disabled. LASS is also disabled.
+
+KVM and PVM specific CPUID leafs
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+- When CPUID.EAX = KVM_CPUID_SIGNATURE (0x40000000) is entered, the
+  output CPUID.EAX will be at least 0x40000002 which is
+  KVM_CPUID_VENDOR_FEATURES (iff the hypervisor is a PVM hypervisor).
+- When CPUID.EAX = KVM_CPUID_VENDOR_FEATURES(0x40000002) is entered,
+  the output CPUID.EAX is PVM features; CPUID.EBX is 0x6d7670 ("pvm");
+  CPUID.ECX and CPUID.EDX are reserved (0).
+
+PVM booting sequence
+^^^^^^^^^^^^^^^^^^^^
+
+The PVM supervisor software has to relocate itself to conform its
+allowed address ranges (See MSR_PVM_LINEAR_ADDRESS_RANGE) and prepare
+itself for its special event handling mechanism on booting.
+
+PVM software can be booted via linux general booting entry points, so
+the software must detect whether itself is PVM as early as possible.
+
+Booting sequence for detecting PVM in 64 bit linux general booting entry:
+
+- check if the underlying EFLAGS.IF is 1
+- check if the underlying CS.CPL is 3
+- use the synthetic instruction KVM_CPUID to check KVM_CPUID_SIGNATURE
+  and KVM_CPUID_VENDOR_FEATURES including checking the signature.
+
+PVM is the first to define such booting sequence, so any later paravirt
+hypervisor that can boot a 64 bit linux guest with underlying
+EFLAGS.IF==1 and CS.CPL == 3 from the linux general booting entry points
+should support the synthetic instruction KVM_CPUID for compatibility.
+
+.. paging:
+
+Paging
+------
+
+PVM MMU has two registers for pagetables: ``CR3`` and ``MSR_PVM_SWITCH_CR3``
+and they are automatically switched on switching user/supervisor modes.
+When in supervisor mode, ``CR3`` holds the kernel pagetable and
+``MSR_PVM_SWITCH_CR3`` holds the user pagetable. These two pagetables work
+in the same way as the two pagetables for KPTI.
+
+The U/S bit in the paging struct is not always honored in PVM and is
+sometimes ignored. User mode software may or may not access the final
+page even if it is a supervisor page (in the view of X86). In fact, due
+to the lack of legacy segment-based isolation, both the user page and
+kernel page in PVM are shadowed as user pages in the underlying
+pagetable with only hypervisor pages with the U bit cleared in the
+underlying pagetable.
+
+It is recommended to have no supervisor pages in the user pagetable. (To
+make more use of the existing KPTI code, this rule can be relaxed as "it
+is recommended that any paging tree should be all supervisor pages or
+all user pages in the user pagetable except for the root PGD
+pagetable.")
+
+And the lack of legacy segment-based isolation is also the reason why
+PVM has two registers for pagetables and the automatically switching
+feature.
+
+Due to the ignoring U/S bit, some features are disabled in PVM.
+
+- SMEP is disabled and ``CR4.SMEP`` can not be set. The guest can use
+  the NX bit for the user pages in the supervisor pagetable to regain
+  the protection.
+
+- SMAP is disabled and ``CR4.SMAP`` can not be set. The guest can
+  emulate it via PKS.
+
+- PKS feature is changed. Protection Key protection doesn't consider
+  the U/S bit, it protects all the data access based on the key. The
+  software should distribute different keys for supervisor pages and
+  user pages.
+
+TLB
+~~~
+
+| TLB entries are considered to be tagged by the root page table (PGD)
+  pointer.
+
+- Hypercall HC_TLB_FLUSH_CURENT, HC_TLB_FLUSH, and HC_TLB_LOAD_PGTBL
+  flush TLB entries based on the tags (PGD of ``CR3`` and
+  ``MSR_PVM_SWITCH_CR3``).
+- ``CR3`` and ``MSR_PVM_SWITCH_CR3`` are swapped on switching
+  user/supervisor mode but no TLB flushing is performed.
+- Writing to ``CR3`` may not flush TLB for ``MSR_PVM_SWITCH_CR3``.
+- WRMSR or HC_WRMSR to ``MSR_PVM_SWITCH_CR3`` doesn't flush TLB.
+- ``CR4.PCID`` bit is recommended to be set even if the
+  ``underlying CR4.PCID`` is cleared so that the PVM TLB can be flushed
+  only on demand.
+
+Exclusive address ranges
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+A portion of the upper half of the linear address is separated from
+the host kernel and the host doesn't use this separated portion. Only
+the address in this separated portion and the lower half is the
+guest-allowed linear address.
+
+.. protection-keys:
+
+Protection Keys
+~~~~~~~~~~~~~~~
+
+There are no distinctions between PVM user pages and PVM supervisor
+pages in the real hardware. Protection Keys protection protects all data
+accesses if enabled. ``CR4.PKE`` enables Protection Keys protection in
+user mode while ``CR4.PKS`` enables Protection Keys protection in
+supervisor mode.
+
+``CR4.PKS`` can only be enabled when ``CR4.PKE`` is enabled and
+``CR4.PKE`` can only be enabled when the underlying ``CR4.PKE`` is
+enabled.
+
+The ``underlying PKRU`` is the effective protection key register in both
+supervisor mode and user mode.
+
+The supervisor software should distribute different keys for supervisor
+mode and user mode so that the PVM ``PKRU`` and ``MSR_IA32_PKRS``\ (in
+guest supervisor view) are mapped to the different parts of the
+``underlying PKRU`` at the same time. With distributed different keys,
+``SUPERVISOR_KEYS_MASK`` can be defined in the guest supervisor.
+
+- The ``MSR_IA32_PKRS`` (in guest supervisor view) is the
+  ``underlying PKRU`` masked with ``SUPERVISOR_KEYS_MASK``, and it is
+  invisible to the hypervisor since ``SUPERVISOR_KEYS_MASK`` is
+  invisible to the hypervisor.
+- ``MSR_IA32_PKRS`` (in hypervisor view) is recommended to be set as the
+  same as ``MSR_IA32_PKRS`` (in guest supervisor view) before returning
+  to the user mode so that after the next switchback, the user part of
+  the ``underlying PKRU`` is access-denied and the supervisor part is
+  already set properly.
+
+If host/hardware ``CR4.PKE`` is set: the hypervisor/switcher will do
+these no matter what the value of ``CR4.PKE`` or ``CR4.PKS:``
+
+- supervisor -> user switching: load the ``underlying PKRU`` with
+  ``PVCS::pkru``
+
+- user -> supervisor switching: save the ``underlying PKRU`` to
+  ``PVCS::pkru``\ ， load the ``underlying PKRU`` with a default value
+  (0 or ``MSR_IA32_PKRS`` if ``CR4.PKS``).
+
+SMAP
+~~~~
+
+| PVM doesn't support SMAP, if the guest supervisor wants to protect
+  user access, it should use ``CR4.PKS``.
+
+- The software should distribute different keys for supervisor mode and
+  user mode.
+- ``MSR_IA32_PKRS`` should be set with the user keys as access-denied.
+- Events handlers in supervisor mode
+
+  - Save the old ``underlying PKRU`` and set it to ``MSR_IA32_PKRS`` on entry
+    so that the user part of the ``underlying PKRU`` is access-denied.
+  - Restore the ``underlying PKRU`` on exit.
+
+- When accessing to 'PVM user page' in supervisor mode
+
+  - Set the ``underlying PKRU`` to (``MSR_IA32_PKRS`` &
+    ``SUPERVISOR_KEYS_MASK``) \| ``PVCS::pkru``
+  - Restore the ``underlying PKRU`` when after it finishes the access.
+
+
+Events and Mode Changing
+------------------------
+
+Special Events
+~~~~~~~~~~~~~~
+
+No DoubleFault
+^^^^^^^^^^^^^^
+
+#DF is always promoted to TripleFault and brings down the PVM instance.
+
+Discarded #DB
+^^^^^^^^^^^^^
+
+When MOV/POP SS from a watched address is followed by any
+instruction-trap-induced supervisor mode entries, the MOV/POP SS that
+hits the watchpoint will be discarded instead.
+
+Vector events in user mode
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+When vector events happen in user mode, the hypervisor is responsible
+for saving guest registers into ``PVCS``, including ``SS``, ``CS``,
+``PKRU``, ``GSBASE``, ``RSP``, ``RFLAGS``, ``RIP``, ``RCX``, and
+``R11``.
+
+The PVM hypervisor should also save the event vector into
+``PVCS::event_vector`` and the error code in ``PVCS::event_errcode``,
+and ``CR2`` into ``PVCS::cr2`` if it is pagefault event.
+
+No change to ``PVCS::event_flags.IF``\ (bit 9) during delivering any
+event in user mode, and the supervisor software is recommended to ensure
+it unset.
+
+Before returning to the guest supervisor, the PVM hypervisor will also
+load values to vCPU with the following actions:
+
+- Inexplicitly load ``CS/SS`` with the value the supervisor expects
+  from ``MSR_STAR``.
+
+  - The ``underlying CS/SS`` is loaded with host-defined ``__USER_CS``
+    and ``__USER_DS``.
+
+- Switch ``CR3`` with ``MSR_PVM_SWITCH_CR3`` without flushing TLB
+
+  - The ``underlying CR3`` is the actual shadow root page table for
+    the new ``PVM CR3``.
+
+- Load ``GSBASE`` with ``MSR_KERNEL_GS_BASE``.
+
+- Load ``RSP`` with ``MSR_PVM_KERNEL_RSP``.
+
+- Load ``RIP/RCX`` with ``MSR_PVM_EVENT_ENTRY``.
+
+- Load ``R11`` with (``X86_EFLAGS_IF`` \| ``X86_EFLAGS_FIXED``).
+
+- Load ``RFLAGS`` with ``X86_EFLAGS_FIXED``.
+
+  - The ``underlying RFLAGS`` is the same as ``R11`` which is
+    (``X86_EFLAGS_IF`` \| ``X86_EFLAGS_FIXED``).
+
+Vector events in supervisor mode
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The hypervisor handles vector events differently based on the vector
+and there is no IST stacks.
+
+The hypervisor handles vector events occurring in supervisor mode with
+vector number < 32 as these uninterruptible steps:
+
+- Subtract the fixed size (MSR_PVM_SUPERVISOR_REDZONE) from RSP.
+- Align RSP down to a 16-byte boundary.
+- Push R11
+- Push Rcx
+- Push SS
+- Push original RSP
+- Push RFLAGS
+
+  - ``RFLAGS.IF`` comes from ``PVCS::event_flags.IF`` (bit 9),
+     which means the pushed ``RFLAGS`` is ``(underlying RFLAGS ~
+     X86_EFLAGS_IF) | (PVCS::event_flags & X86_EFLAGS_IF)``
+
+- Push CS
+- Push RIP
+- Push vector (4 bytes), ERRCODE (4 bytes)
+- If it is pagefault, save CR2 into PVCS:cr2
+- No change to ``CS/SS.``
+- Load ``RSP`` with the result after the last push as described above.
+- Load ``R11`` with (``X86_EFLAGS_IF`` \| ``X86_EFLAGS_FIXED``).
+- Load ``RFLAGS`` with ``X86_EFLAGS_FIXED``.
+
+  - The ``underlying RFLAGS`` is the same as ``R11`` which is
+    (``X86_EFLAGS_IF`` \| ``X86_EFLAGS_FIXED``).
+  - PVCS::event_flags.IF will be cleared if it is previously set.
+
+- Load ``RIP/RCX`` with ``MSR_PVM_EVENT_ENTRY``\ +256
+
+The hypervisor handles vector events occurring in supervisor mode with
+vector number => 32 as these uninterruptible steps:
+
+- Save R11,RCX,RSP,EFLAGS,RIP to PVCS.
+- Save the vector number to PVCS:event_vector.
+- No change to ``CS/SS.``
+- Subtract the fixed size (MSR_PVM_SUPERVISOR_REDZONE) from RSP.
+- Load RSP with the current RSP value aligned down to a 16-byte boundary.
+- Load ``R11`` with (``X86_EFLAGS_IF`` \| ``X86_EFLAGS_FIXED``).
+- Load ``RFLAGS`` with ``X86_EFLAGS_FIXED``
+
+  - The ``underlying RFLAGS`` is the same as ``R11`` which is
+    (``X86_EFLAGS_IF`` \| ``X86_EFLAGS_FIXED``).
+  - PVCS::event_flags.IF will be cleared if it is previously set.
+
+- Load ``RIP/RCX`` with ``MSR_PVM_EVENT_ENTRY``\ +512
+
+User SYSCALL event
+~~~~~~~~~~~~~~~~~~
+
+SYSCALL instruction in PVM user mode is a user SYSCALL event and the
+hypervisor handles it almost as the same as vector events in user mode
+except that no change to ``PVCS::event_vector``, ``PVCS::event_errcode``
+and ``PVCS::cr2`` and ``RIP/RCX`` is loaded with ``MSR_LSTAR``.
+
+Specifically, the hypervisor saves guest registers into ``PVCS``,
+including ``SS``, ``CS``, ``PKRU``, ``GSBASE``, ``RSP``, ``RFLAGS``,
+RIP, ``RCX``, and ``R11``, and loads values to vCPU with the following
+actions:
+
+- Inexplicitly load ``CS/SS`` with the value the supervisor expects
+  from ``MSR_STAR``.
+
+  - The ``underlying CS/SS`` is loaded with host-defined ``__USER_CS``
+    and ``__USER_DS``.
+
+- Switch ``CR3`` with ``MSR_PVM_SWITCH_CR3`` without flushing TLB
+
+  - The ``underlying CR3`` is the actual shadow root page table for
+    the new ``PVM CR3``.
+
+- Load ``GSBASE`` with ``MSR_KERNEL_GS_BASE``.
+- Load ``RSP`` with ``MSR_PVM_KERNEL_RSP``.
+- Load ``RIP/RCX`` with ``MSR_LSTAR``.
+- Load ``R11`` with (``X86_EFLAGS_IF`` \| ``X86_EFLAGS_FIXED``).
+- Load ``RFLAGS`` with ``X86_EFLAGS_FIXED``.
+
+  - The ``underlying RFLAGS`` is the same as ``R11`` which is
+    (``X86_EFLAGS_IF`` \| ``X86_EFLAGS_FIXED``).
+  - No change to ``PVCS::event_flags.IF``\ (bit 9) during delivering
+    the SYSCALL event, and the supervisor software is recommended to
+    ensure it unset.
+
+
+Synthetic Instruction: EVENT_RETURN_USER
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+This synthetic instruction is the only way for the PVM supervisor to
+switch to user mode.
+
+It works as the opposite operations of the event in user mode: load
+``CS``, ``SS``, ``GSBASE``, ``PKRU``, ``RSP``, ``RFLAGS``, RIP, ``RCX``,
+and ``R11`` from the ``PVCS`` respectively with some conversions to
+``GSBASE`` and ``RFLAGS``; switch ``CR3`` and ``MSR_PVM_SWITCH_CR3`` and
+return to user mode. The origian ``RSP`` is saved into
+``MSR_PVM_SUPERVISOR_RSP``.
+
+No change to ``PVCS::event_flags.IF``\ (bit 9) during handling it
+and the supervisor software is recommended to ensure it unset.
+
+Synthetic Instruction: EVENT_RETURN_SUPERVISOR
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+| Return to PVM supervisor mode.
+| Work almost the same as IRETQ instruction except for ``RCX``, ``R11`` and
+  ``ERRCODE`` are also in the stack.
+
+It expects the stack frame:
+
+.. code::
+
+   R11
+   RCX
+   SS
+   RSP
+   RFLAGS
+   CS
+   RIP
+   ERRCODE
+
+Return to the context with RIP, RFLAGS, RSP, RCX, and R11 restored from the
+stack.
+
+The ``CS/SS`` and ``ERRCODE`` in the stack are ignored and the current PVM
+``CS/SS`` are unchanged.
+
+Hypercall event in supervisor mode
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Except for the synthetic instructions, SYSCALL instructions in PVM
+supervisor mode is a HYPERCALL.
+
+``RAX`` is the request number of the HYPERCALL. Some hypercall request
+numbers are PVM-specific HYPERCALLs. Other values are KVM-specific
+HYPERCALL.
+
+HYPERCALL be issued in supervisor software
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+PVM supervisor software saves ``R10``, ``R11`` onto the stack and copies
+``RCX`` into ``R10``, and then invokes the SYSCALL instruction. After
+the HYPERCALL(SYSCALL instruction) returns, the software should get
+``RCX`` from ``R10`` and restore ``R10`` and ``R11`` from the stack.
+
+Hypercall's behavior should treat ``R10`` as ``RCX`` (in PVM
+hypervisor):
+
+.. code::
+
+   RCX := R10
+   pvm or kvm hypercall handling.
+   R10 := RCX
+
+If not specific, the return result is in ``RAX``.
+
+PVM_HC_LOAD_PGTBL
+^^^^^^^^^^^^^^^^^
+
+| Parameters: *flags*, *supervisor_pgd*, *user_pgd*.
+| Loads the pagetables
+|  \* flags bit0: flush the new supervisor_pgd and user_pgd.
+|  \* flags bit1: 4-level(bit1=0) or 5-level(bit1=1 && LA57 is supported
+  in the VCPU's cpuid features) pagetable, the ``CR4.LA57`` bit is also
+  changed correspondingly.
+|  \* supervisor_pgd: set to ``CR3``
+|  \* user_pgd: set to ``MSR_PVM_SWITCH_CR3``
+
+PVM_HC_IRQ_WIN
+^^^^^^^^^^^^^^
+
+| No parameters.
+| Infos the hypervisor that IRQ is enabled.
+
+PVM_HC_IRQ_HLT
+^^^^^^^^^^^^^^
+
+| No parameters.
+| Emulates the combination of X86 instructions "STI; HLT;".
+
+PVM_HC_TLB_FLUSH
+^^^^^^^^^^^^^^^^
+
+| No parameters.
+| Flush all TLB
+
+PVM_HC_TLB_FLUSH_CURRENT
+^^^^^^^^^^^^^^^^^^^^^^^^
+
+| No parameters.
+| Flush the TLB associated with the current ``PVM CR3`` and
+  ``MSR_PVM_SWITCH_CR3``.
+
+PVM_HC_TLB_INVLPG
+^^^^^^^^^^^^^^^^^
+
+| Parameters: *addr*.
+| Emulates INVLPG and Flush the TLB entries of the address.
+
+PVM_HC_LOAD_GS
+^^^^^^^^^^^^^^
+
+| Parameters: *gs_sel*.
+| Load GS with the selector gs_sel, if it fails, load GS with the NULL
+  selector.
+| Return the resulting GS_BASE.
+
+PVM_HC_RDMSR
+^^^^^^^^^^^^
+
+| Parameters: msr_index
+| Returns the MSR value or zero if the MSR index is invalid
+
+PVM_HC_WRMSR
+^^^^^^^^^^^^
+
+| Parameters: msr_index, msr_value
+| return 0 or -EINVAL.
+
+PVM_HC_LOAD_TLS
+^^^^^^^^^^^^^^^
+
+| Parameters: gdt_entry0, gdt_entry1, gdt_entry2
+| Rectify gdt_entry0, gdt_entry1, and gdt_entry2 and set them
+  continuously in the HOST ``GDT``.
+| Return HOST ``GDT`` index for *gdt_entry0*.
