From: Janis Schoetterl-Glausch <scgl@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> Reviewed-by: Janosch Frank <frankja@xxxxxxxxxxxxx> Link: https://lore.kernel.org/r/20230206164602.138068-14-scgl@xxxxxxxxxxxxx Message-Id: <20230206164602.138068-14-scgl@xxxxxxxxxxxxx> [frankja@xxxxxxxxxx: Removed a line from an earlier version] Signed-off-by: Janosch Frank <frankja@xxxxxxxxxxxxx> --- Documentation/virt/kvm/api.rst | 28 +++++++++++++++++++++++++--- 1 file changed, 25 insertions(+), 3 deletions(-) diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst index 8cd7fd05d53b..a4c9dbccc7c2 100644 --- 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: ^^^^^^^^^^^^^^^^ -- 2.39.1
