[GIT,PULL,15/18] Documentation: KVM: s390: Describe KVM_S390_MEMOP_F_CMPXCHG

Commit Message

Janosch Frank Feb. 9, 2023, 10:22 a.m. UTC

From: Janis Schoetterl-Glausch <[e-mail]>

Describe the semantics of the new KVM_S390_MEMOP_F_CMPXCHG flag for
absolute vm write memops which allows user space to perform (storage key
checked) cmpxchg operations on guest memory.

Signed-off-by: Janis Schoetterl-Glausch <[e-mail]>
Reviewed-by: Janosch Frank <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Message-Id: <[e-mail]>
[[e-mail]: Removed a line from an earlier version]
Signed-off-by: Janosch Frank <[e-mail]>
---
 Documentation/virt/kvm/api.rst | 28 +++++++++++++++++++++++++---
 1 file changed, 25 insertions(+), 3 deletions(-)

Patch

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 8cd7fd05d53b..a4c9dbccc7c2 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -3728,7 +3728,7 @@  The fields in each entry are defined as follows:
 :Parameters: struct kvm_s390_mem_op (in)
 :Returns: = 0 on success,
           < 0 on generic error (e.g. -EFAULT or -ENOMEM),
-          > 0 if an exception occurred while walking the page tables
+          16 bit program exception code if the access causes such an exception
 
 Read or write data from/to the VM's memory.
 The KVM_CAP_S390_MEM_OP_EXTENSION capability specifies what functionality is
@@ -3746,6 +3746,8 @@  Parameters are specified via the following structure::
 		struct {
 			__u8 ar;	/* the access register number */
 			__u8 key;	/* access key, ignored if flag unset */
+			__u8 pad1[6];	/* ignored */
+			__u64 old_addr;	/* ignored if flag unset */
 		};
 		__u32 sida_offset; /* offset into the sida */
 		__u8 reserved[32]; /* ignored */
@@ -3773,6 +3775,7 @@  Possible operations are:
   * ``KVM_S390_MEMOP_ABSOLUTE_WRITE``
   * ``KVM_S390_MEMOP_SIDA_READ``
   * ``KVM_S390_MEMOP_SIDA_WRITE``
+  * ``KVM_S390_MEMOP_ABSOLUTE_CMPXCHG``
 
 Logical read/write:
 ^^^^^^^^^^^^^^^^^^^
@@ -3821,7 +3824,7 @@  the checks required for storage key protection as one operation (as opposed to
 user space getting the storage keys, performing the checks, and accessing
 memory thereafter, which could lead to a delay between check and access).
 Absolute accesses are permitted for the VM ioctl if KVM_CAP_S390_MEM_OP_EXTENSION
-is > 0.
+has the KVM_S390_MEMOP_EXTENSION_CAP_BASE bit set.
 Currently absolute accesses are not permitted for VCPU ioctls.
 Absolute accesses are permitted for non-protected guests only.
 
@@ -3829,7 +3832,26 @@  Supported flags:
   * ``KVM_S390_MEMOP_F_CHECK_ONLY``
   * ``KVM_S390_MEMOP_F_SKEY_PROTECTION``
 
-The semantics of the flags are as for logical accesses.
+The semantics of the flags common with logical accesses are as for logical
+accesses.
+
+Absolute cmpxchg:
+^^^^^^^^^^^^^^^^^
+
+Perform cmpxchg on absolute guest memory. Intended for use with the
+KVM_S390_MEMOP_F_SKEY_PROTECTION flag.
+Instead of doing an unconditional write, the access occurs only if the target
+location contains the value pointed to by "old_addr".
+This is performed as an atomic cmpxchg with the length specified by the "size"
+parameter. "size" must be a power of two up to and including 16.
+If the exchange did not take place because the target value doesn't match the
+old value, the value "old_addr" points to is replaced by the target value.
+User space can tell if an exchange took place by checking if this replacement
+occurred. The cmpxchg op is permitted for the VM ioctl if
+KVM_CAP_S390_MEM_OP_EXTENSION has flag KVM_S390_MEMOP_EXTENSION_CAP_CMPXCHG set.
+
+Supported flags:
+  * ``KVM_S390_MEMOP_F_SKEY_PROTECTION``
 
 SIDA read/write:
 ^^^^^^^^^^^^^^^^