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

[Thomas Huth]

On 30/09/2022 23.07, 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@xxxxxxxxxxxxx>
> ---
>   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 */

Padding / alignment?

> +			__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.

Please try to fit the text within 80 columns.

 Thanks,
  Thomas