[PATCH v3 2/9] Documentation: KVM: s390: Describe KVM_S390_MEMOP_F_CMPXCHG
From: Janis Schoetterl-Glausch <hidden>
Date: 2022-11-17 22:19:49
Also in:
kvm, linux-kselftest, linux-s390, lkml
Subsystem:
documentation, kernel virtual machine (kvm), the rest · Maintainers:
Jonathan Corbet, Paolo Bonzini, Linus Torvalds
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 <redacted> --- 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 eee9f857a986..204d128f23e0 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: ^^^^^^^^^^^^^^^^
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2.34.1