Re: [PATCH v3 2/9] Documentation: KVM: s390: Describe KVM_S390_MEMOP_F_CMPXCHG

Thomas Huth
2022-11-22T07:48:35Z

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 <scgl@linux.ibm.com>
> ---
...
> 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.