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 + other code > 0xffff with special meaning
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: ^^^^^^^^^^^^^^^^