| Message ID | 20230914044805.301390-7-xin3.li@intel.com (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
| Series | x86: enable FRED for x86-64 | expand |
On 14.09.23 г. 7:47 ч., Xin Li wrote: > Briefly introduce FRED, and its advantages compared to IDT. > > Signed-off-by: Xin Li <xin3.li@intel.com> > --- > Documentation/arch/x86/x86_64/fred.rst | 98 +++++++++++++++++++++++++ > Documentation/arch/x86/x86_64/index.rst | 1 + > 2 files changed, 99 insertions(+) > create mode 100644 Documentation/arch/x86/x86_64/fred.rst > > diff --git a/Documentation/arch/x86/x86_64/fred.rst b/Documentation/arch/x86/x86_64/fred.rst > new file mode 100644 > index 000000000000..a4ebb95f92c8 > --- /dev/null > +++ b/Documentation/arch/x86/x86_64/fred.rst > @@ -0,0 +1,98 @@ > +.. SPDX-License-Identifier: GPL-2.0 > + > +========================================= > +Flexible Return and Event Delivery (FRED) > +========================================= > + > +Overview > +======== > + > +The FRED architecture defines simple new transitions that change > +privilege level (ring transitions). The FRED architecture was > +designed with the following goals: > + > +1) Improve overall performance and response time by replacing event > + delivery through the interrupt descriptor table (IDT event > + delivery) and event return by the IRET instruction with lower > + latency transitions. > + > +2) Improve software robustness by ensuring that event delivery > + establishes the full supervisor context and that event return > + establishes the full user context. > + > +The new transitions defined by the FRED architecture are FRED event > +delivery and, for returning from events, two FRED return instructions. > +FRED event delivery can effect a transition from ring 3 to ring 0, but > +it is used also to deliver events incident to ring 0. One FRED > +instruction (ERETU) effects a return from ring 0 to ring 3, while the > +other (ERETS) returns while remaining in ring 0. Collectively, FRED > +event delivery and the FRED return instructions are FRED transitions. > + > +In addition to these transitions, the FRED architecture defines a new > +instruction (LKGS) for managing the state of the GS segment register. > +The LKGS instruction can be used by 64-bit operating systems that do > +not use the new FRED transitions. > + > +Furthermore, the FRED architecture is easy to extend for future CPU > +architectures. > + > +Software based event dispatching > +================================ > + > +FRED operates differently from IDT in terms of event handling. Instead > +of directly dispatching an event to its handler based on the event > +vector, FRED requires the software to dispatch an event to its handler > +based on both the event's type and vector. Therefore, an event dispatch > +framework must be implemented to facilitate the event-to-handler > +dispatch process. The FRED event dispatch framework takes control > +once an event is delivered, and employs a two-level dispatch. > + > +The first level dispatching is event type based, and the second level > +dispatching is event vector based. > + > +Full supervisor/user context > +============================ > + > +FRED event delivery atomically save and restore full supervisor/user > +context upon event delivery and return. Thus it avoids the problem of > +transient states due to %cr2 and/or %dr6, and it is no longer needed > +to handle all the ugly corner cases caused by half baked entry states. > + > +FRED allows explicit unblock of NMI with new event return instructions > +ERETS/ERETU, avoiding the mess caused by IRET which unconditionally > +unblocks NMI, e.g., when an exception happens during NMI handling. > + > +FRED always restores the full value of %rsp, thus ESPFIX is no longer > +needed when FRED is enabled. > + > +LKGS > +==== > + > +LKGS behaves like the MOV to GS instruction except that it loads the > +base address into the IA32_KERNEL_GS_BASE MSR instead of the GS > +segment’s descriptor cache. With LKGS, it ends up with avoiding > +mucking with kernel GS, i.e., an operating system can always operate > +with its own GS base address. > + > +Because FRED event delivery from ring 3 swaps the value of the GS base > +address and that of the IA32_KERNEL_GS_BASE MSR, and ERETU swaps the > +value of the GS base address and that of the IA32_KERNEL_GS_BASE MSR, > +plus the introduction of LKGS instruction, the SWAPGS instruction is > +no longer needed when FRED is enabled, thus is disallowed (#UD). nit: This will be more clear if rewritten: "Because FRED event delivery from ring 3 and ERETU both swap the value of the GS base, plus the..." . The idea is to remove the duplicate statement that IA32_KERNEL_GS_BASE and the GS registers are swapped as it makes the sentence somewhat hard to read. <snip>
diff --git a/Documentation/arch/x86/x86_64/fred.rst b/Documentation/arch/x86/x86_64/fred.rst new file mode 100644 index 000000000000..a4ebb95f92c8 --- /dev/null +++ b/Documentation/arch/x86/x86_64/fred.rst @@ -0,0 +1,98 @@ +.. SPDX-License-Identifier: GPL-2.0 + +========================================= +Flexible Return and Event Delivery (FRED) +========================================= + +Overview +======== + +The FRED architecture defines simple new transitions that change +privilege level (ring transitions). The FRED architecture was +designed with the following goals: + +1) Improve overall performance and response time by replacing event + delivery through the interrupt descriptor table (IDT event + delivery) and event return by the IRET instruction with lower + latency transitions. + +2) Improve software robustness by ensuring that event delivery + establishes the full supervisor context and that event return + establishes the full user context. + +The new transitions defined by the FRED architecture are FRED event +delivery and, for returning from events, two FRED return instructions. +FRED event delivery can effect a transition from ring 3 to ring 0, but +it is used also to deliver events incident to ring 0. One FRED +instruction (ERETU) effects a return from ring 0 to ring 3, while the +other (ERETS) returns while remaining in ring 0. Collectively, FRED +event delivery and the FRED return instructions are FRED transitions. + +In addition to these transitions, the FRED architecture defines a new +instruction (LKGS) for managing the state of the GS segment register. +The LKGS instruction can be used by 64-bit operating systems that do +not use the new FRED transitions. + +Furthermore, the FRED architecture is easy to extend for future CPU +architectures. + +Software based event dispatching +================================ + +FRED operates differently from IDT in terms of event handling. Instead +of directly dispatching an event to its handler based on the event +vector, FRED requires the software to dispatch an event to its handler +based on both the event's type and vector. Therefore, an event dispatch +framework must be implemented to facilitate the event-to-handler +dispatch process. The FRED event dispatch framework takes control +once an event is delivered, and employs a two-level dispatch. + +The first level dispatching is event type based, and the second level +dispatching is event vector based. + +Full supervisor/user context +============================ + +FRED event delivery atomically save and restore full supervisor/user +context upon event delivery and return. Thus it avoids the problem of +transient states due to %cr2 and/or %dr6, and it is no longer needed +to handle all the ugly corner cases caused by half baked entry states. + +FRED allows explicit unblock of NMI with new event return instructions +ERETS/ERETU, avoiding the mess caused by IRET which unconditionally +unblocks NMI, e.g., when an exception happens during NMI handling. + +FRED always restores the full value of %rsp, thus ESPFIX is no longer +needed when FRED is enabled. + +LKGS +==== + +LKGS behaves like the MOV to GS instruction except that it loads the +base address into the IA32_KERNEL_GS_BASE MSR instead of the GS +segment’s descriptor cache. With LKGS, it ends up with avoiding +mucking with kernel GS, i.e., an operating system can always operate +with its own GS base address. + +Because FRED event delivery from ring 3 swaps the value of the GS base +address and that of the IA32_KERNEL_GS_BASE MSR, and ERETU swaps the +value of the GS base address and that of the IA32_KERNEL_GS_BASE MSR, +plus the introduction of LKGS instruction, the SWAPGS instruction is +no longer needed when FRED is enabled, thus is disallowed (#UD). + +Stack levels +============ + +4 stack levels 0~3 are introduced to replace the nonreentrant IST for +event handling, and each stack level should be configured to use a +dedicated stack. + +The current stack level could be unchanged or go higher upon FRED +event delivery. If unchanged, the CPU keeps using the current event +stack. If higher, the CPU switches to a new event stack specified by +the MSR of the new stack level, i.e., MSR_IA32_FRED_RSP{1,2,3}. + +Only execution of a FRED return instruction ERET{U,S}, could lower +the current stack level, causing the CPU to switch back to the stack +it was on before a previous event delivery that promoted the stack +level. diff --git a/Documentation/arch/x86/x86_64/index.rst b/Documentation/arch/x86/x86_64/index.rst index a56070fc8e77..ad15e9bd623f 100644 --- a/Documentation/arch/x86/x86_64/index.rst +++ b/Documentation/arch/x86/x86_64/index.rst @@ -15,3 +15,4 @@ x86_64 Support cpu-hotplug-spec machinecheck fsgs + fred
Briefly introduce FRED, and its advantages compared to IDT. Signed-off-by: Xin Li <xin3.li@intel.com> --- Documentation/arch/x86/x86_64/fred.rst | 98 +++++++++++++++++++++++++ Documentation/arch/x86/x86_64/index.rst | 1 + 2 files changed, 99 insertions(+) create mode 100644 Documentation/arch/x86/x86_64/fred.rst