On Tue, 13 Dec 2022 17:53:58 +0100
Janis Schoetterl-Glausch <scgl@xxxxxxxxxxxxx> 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 | 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@xxxxxxxxxxxxx>
>
> 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:
> ^^^^^^^^^^^^^^^^
