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

Claudio Imbrenda
2022-12-01T16:39:22Z

On Thu, 17 Nov 2022 23:17:51 +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 | 21 +++++++++++++++++++--
> 1 file changed, 19 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index eee9f857a986..204d128f23e0 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
> + other code > maximum 16 bit value with special meaning

I would write the number explicitly ( > 65535 or > 0xffff )

>
> 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_p; /* ignored if flag unset */
> };
> __u32 sida_offset; /* offset into the sida */
> __u8 reserved[32]; /* ignored */
> @@ -3853,8 +3856,22 @@ 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 "size" byte long value pointed to by
> +"old_p". This is performed as an atomic cmpxchg. "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.
> +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.
>
> -The semantics of the flags are as for logical accesses.
>
> SIDA read/write:
> ^^^^^^^^^^^^^^^^