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 exceptionother 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 */@@ -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: ^^^^^^^^^^^^^^^^