On 17/11/2022 23.17, Janis Schoetterl-Glausch wrote:

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 <redacted>
---

...

  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.

I'd maybe merge this with the last sentence:

For write accesses, 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.

... and speaking of that, I wonder whether it's maybe a good idea to 
introduce some #defines for bit 1 / value 2, to avoid the confusion ?

+In this case, instead of doing an unconditional write, the access occurs only
+if the target location contains the "size" byte long value pointed to by
+"old_p". This is performed as an atomic cmpxchg.

I had to read the first sentence twice to understand it ... maybe it's 
easier to understand if you move the "size" part to the second sentence:

In this case, instead of doing an unconditional write, the access occurs 
only if the target location contains value pointed to by "old_p". 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.
+The value at the target location is written to the location "old_p" points to.

IMHO something like this would be better:

The value at the target location is replaced with the value from the 
location that "old_p" points to.

+If the exchange did not take place because the target value doesn't match the
+old value KVM_S390_MEMOP_R_NO_XCHG is returned.
+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.

  Thomas

PS: Please take my suggestions with a grain of salt ... I'm not a native 
speaker either.