Documentation: KVM: s390: Describe KVM_S390_MEMOP_F_CMPXCHG

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>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Link: https://lore.kernel.org/r/20230206164602.138068-14-scgl@linux.ibm.com
Message-Id: <20230206164602.138068-14-scgl@linux.ibm.com>
[frankja@de.ibm.com: Removed a line from an earlier version]
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 8cd7fd05d53b5739917e62127bd828468b71f7b4..a4c9dbccc7c23c138a06afff108a2048a5a6b20c 100644 (file)
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -3728,7 +3728,7 @@ 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
 
 Read or write data from/to the VM's memory.
 The KVM_CAP_S390_MEM_OP_EXTENSION capability specifies what functionality is
@@ -3746,6 +3746,8 @@ 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 */
@@ -3773,6 +3775,7 @@ Possible operations are:
   * ``KVM_S390_MEMOP_ABSOLUTE_WRITE``
   * ``KVM_S390_MEMOP_SIDA_READ``
   * ``KVM_S390_MEMOP_SIDA_WRITE``
+  * ``KVM_S390_MEMOP_ABSOLUTE_CMPXCHG``
 
 Logical read/write:
 ^^^^^^^^^^^^^^^^^^^
@@ -3821,7 +3824,7 @@ the checks required for storage key protection as one operation (as opposed to
 user space getting the storage keys, performing the checks, and accessing
 memory thereafter, which could lead to a delay between check and access).
 Absolute accesses are permitted for the VM ioctl if KVM_CAP_S390_MEM_OP_EXTENSION
-is > 0.
+has the KVM_S390_MEMOP_EXTENSION_CAP_BASE bit set.
 Currently absolute accesses are not permitted for VCPU ioctls.
 Absolute accesses are permitted for non-protected guests only.
 
@@ -3829,7 +3832,26 @@ Supported flags:
   * ``KVM_S390_MEMOP_F_CHECK_ONLY``
   * ``KVM_S390_MEMOP_F_SKEY_PROTECTION``
 
-The semantics of the flags are as for logical accesses.
+The semantics of the flags common with logical accesses are as for logical
+accesses.
+
+Absolute cmpxchg:
+^^^^^^^^^^^^^^^^^
+
+Perform cmpxchg on absolute guest memory. Intended for use with the
+KVM_S390_MEMOP_F_SKEY_PROTECTION flag.
+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, the value "old_addr" points to is replaced by the target value.
+User space can tell if an exchange took place by checking if this replacement
+occurred. The cmpxchg op is permitted for the VM ioctl if
+KVM_CAP_S390_MEM_OP_EXTENSION has flag KVM_S390_MEMOP_EXTENSION_CAP_CMPXCHG set.
+
+Supported flags:
+  * ``KVM_S390_MEMOP_F_SKEY_PROTECTION``
 
 SIDA read/write:
 ^^^^^^^^^^^^^^^^