[PATCH v7 13/14] Documentation: KVM: s390: Describe KVM_S390_MEMOP_F_CMPXCHG

Janis Schoetterl-Glausch
2023-02-06T16:46:45Z

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 | 29 ++++++++++++++++++++++++++---
1 file changed, 26 insertions(+), 3 deletions(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 0a67cb738013..d09d7223c2a6 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -3736,7 +3736,8 @@ 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
@@ -3754,6 +3755,8 @@ 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 */
@@ -3781,6 +3784,7 @@ 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:
^^^^^^^^^^^^^^^^^^^
@@ -3829,7 +3833,7 @@ 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.

@@ -3837,7 +3841,26 @@ 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:
^^^^^^^^^^^^^^^^
--
2.37.2