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@linux.ibm.com
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 */@@ -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