[PATCH v2 3/9] Documentation: KVM: s390: Describe KVM_S390_MEMOP_F_CMPXCHG

[Janis Schoetterl-Glausch <scgl@linux.ibm.com>]

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 abd7c32126ce..f972f5d95e6c 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
 
 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:
 ^^^^^^^^^^^^^^^^
-- 
2.34.1