On Thu, 17 Nov 2022 23:17:51 +0100 Janis Schoetterl-Glausch <scgl@xxxxxxxxxxxxx> wrote: > 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> > --- > 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 I would write the number explicitly ( > 65535 or > 0xffff ) > > 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: > ^^^^^^^^^^^^^^^^