[PATCH v1 3/9] Documentation: KVM: s390: Describe KVM_S390_MEMOP_F_CMPXCHG

[Janis Schoetterl-Glausch <scgl@linux.ibm.com>]

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 <scgl@linux.ibm.com>
---
 Documentation/virt/kvm/api.rst | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index abd7c32126ce..0e02d66e38ae 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -3771,6 +3771,7 @@ Parameters are specified via the following structure::
 		struct {
 			__u8 ar;	/* the access register number */
 			__u8 key;	/* access key, ignored if flag unset */
+			__u64 old[2];	/* ignored if flag unset */
 		};
 		__u32 sida_offset; /* offset into the sida */
 		__u8 reserved[32]; /* ignored */
@@ -3853,8 +3854,23 @@ Absolute accesses are permitted for non-protected guests only.
 Supported flags:
   * ``KVM_S390_MEMOP_F_CHECK_ONLY``
   * ``KVM_S390_MEMOP_F_SKEY_PROTECTION``
+  * ``KVM_S390_MEMOP_F_CMPXCHG``
+
+The semantics of the flags common with logical acesses are as for logical accesses.
+
+For write accesses, the KVM_S390_MEMOP_F_CMPXCHG might be supported.
+In this case, instead of doing an unconditional write, the access occurs only
+if the target location contains the value provided in "old". This is performed
+as an atomic cmpxchg. "size" must be a power of two up to and including 16.
+Values with sizes <8 byte are to be provided by assignment to "old[1]".
+Doublewords are provided with the higher value word in "old[0]" and the lower
+word in "old[1]".
+The value at the target location is returned in "old", encoded in the same manner.
+If the value was exchanged the KVM_S390_MEMOP_F_CMPXCHG bit in "flags" is set to
+0, otherwise it remains set.
+The KVM_S390_MEMOP_F_CMPXCHG flag is supported if KVM_CAP_S390_MEM_OP_EXTENSION
+has bit 1 (i.e. bit with value 2) set.
 
-The semantics of the flags are as for logical accesses.
 
 SIDA read/write:
 ^^^^^^^^^^^^^^^^
-- 
2.34.1