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Documentation: KVM: s390: Describe KVM_S390_MEMOP_F_CMPXCHG

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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>
This commit is contained in:
Janis Schoetterl-Glausch 2023-02-06 17:46:01 +01:00 committed by Janosch Frank
parent 3fd49805d1
commit a7b0417328
1 changed files with 25 additions and 3 deletions
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28
Documentation/virt/kvm/api.rst
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@ -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:
^^^^^^^^^^^^^^^^