Re: [PATCH v4 2/9] Documentation: KVM: s390: Describe KVM_S390_MEMOP_F_CMPXCHG

[Claudio Imbrenda <imbrenda@linux.ibm.com>]

On Tue, 13 Dec 2022 17:53:58 +0100
Janis Schoetterl-Glausch <scgl@linux.ibm.com> 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>
> ---
>  Documentation/virt/kvm/api.rst | 20 ++++++++++++++++++--
>  1 file changed, 18 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index eee9f857a986..98f5a35088b8 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -3753,7 +3753,8 @@ 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

s/$/,/

> +          other code > 0xffff with special meaning

s/$/./

with those two nits fixed:

Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>

>  
>  Read or write data from/to the VM's memory.
>  The KVM_CAP_S390_MEM_OP_EXTENSION capability specifies what functionality is
> @@ -3771,6 +3772,8 @@ 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 */
> @@ -3853,8 +3856,21 @@ 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 accesses are as for logical
> +accesses.
> +
> +For write accesses, the KVM_S390_MEMOP_F_CMPXCHG flag is supported if
> +KVM_CAP_S390_MEM_OP_EXTENSION has flag KVM_S390_MEMOP_EXTENSION_CAP_CMPXCHG set.
> +In this case, 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, KVM_S390_MEMOP_R_NO_XCHG is returned.
> +In this case the value "old_addr" points to is replaced by the target value.
>  
> -The semantics of the flags are as for logical accesses.
>  
>  SIDA read/write:
>  ^^^^^^^^^^^^^^^^