| Message ID | 20220930210751.225873-4-scgl@linux.ibm.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | KVM: s390: Extend MEM_OP ioctl by storage key checked cmpxchg | expand |
On 30/09/2022 23.07, Janis Schoetterl-Glausch 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@linux.ibm.com> > --- > Documentation/virt/kvm/api.rst | 18 +++++++++++++++++- > 1 file changed, 17 insertions(+), 1 deletion(-) > > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst > index abd7c32126ce..0e02d66e38ae 100644 > --- a/Documentation/virt/kvm/api.rst > +++ b/Documentation/virt/kvm/api.rst > @@ -3771,6 +3771,7 @@ Parameters are specified via the following structure:: > struct { > __u8 ar; /* the access register number */ > __u8 key; /* access key, ignored if flag unset */ Padding / alignment? > + __u64 old[2]; /* ignored if flag unset */ > }; > __u32 sida_offset; /* offset into the sida */ > __u8 reserved[32]; /* ignored */ > @@ -3853,8 +3854,23 @@ 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 value provided in "old". This is performed > +as an atomic cmpxchg. "size" must be a power of two up to and including 16. > +Values with sizes <8 byte are to be provided by assignment to "old[1]". > +Doublewords are provided with the higher value word in "old[0]" and the lower > +word in "old[1]". > +The value at the target location is returned in "old", encoded in the same manner. > +If the value was exchanged the KVM_S390_MEMOP_F_CMPXCHG bit in "flags" is set to > +0, otherwise it remains set. > +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. Please try to fit the text within 80 columns. Thanks, Thomas
On Tue, 2022-10-04 at 10:16 +0200, Thomas Huth wrote: > On 30/09/2022 23.07, Janis Schoetterl-Glausch 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@linux.ibm.com> > > --- > > Documentation/virt/kvm/api.rst | 18 +++++++++++++++++- > > 1 file changed, 17 insertions(+), 1 deletion(-) > > > > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst > > index abd7c32126ce..0e02d66e38ae 100644 > > --- a/Documentation/virt/kvm/api.rst > > +++ b/Documentation/virt/kvm/api.rst > > @@ -3771,6 +3771,7 @@ Parameters are specified via the following structure:: > > struct { What is the reason you initially didn't copy the /* in */ comment here? Thanks for the reviews. > > __u8 ar; /* the access register number */ > > __u8 key; /* access key, ignored if flag unset */ > > Padding / alignment? > > > + __u64 old[2]; /* ignored if flag unset */ > > }; > > __u32 sida_offset; /* offset into the sida */ > > __u8 reserved[32]; /* ignored */ > > @@ -3853,8 +3854,23 @@ 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 value provided in "old". This is performed > > +as an atomic cmpxchg. "size" must be a power of two up to and including 16. > > +Values with sizes <8 byte are to be provided by assignment to "old[1]". > > +Doublewords are provided with the higher value word in "old[0]" and the lower > > +word in "old[1]". > > +The value at the target location is returned in "old", encoded in the same manner. > > +If the value was exchanged the KVM_S390_MEMOP_F_CMPXCHG bit in "flags" is set to > > +0, otherwise it remains set. > > +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. > > Please try to fit the text within 80 columns. > > Thanks, > Thomas >
On 04/10/2022 20.51, Janis Schoetterl-Glausch wrote: > On Tue, 2022-10-04 at 10:16 +0200, Thomas Huth wrote: >> On 30/09/2022 23.07, Janis Schoetterl-Glausch 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@linux.ibm.com> >>> --- >>> Documentation/virt/kvm/api.rst | 18 +++++++++++++++++- >>> 1 file changed, 17 insertions(+), 1 deletion(-) >>> >>> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst >>> index abd7c32126ce..0e02d66e38ae 100644 >>> --- a/Documentation/virt/kvm/api.rst >>> +++ b/Documentation/virt/kvm/api.rst >>> @@ -3771,6 +3771,7 @@ Parameters are specified via the following structure:: >>> struct { > > What is the reason you initially didn't copy the /* in */ comment here? You mean in commit 41408c28f283b ? Uh, don't ask me, that's more than 7 years ago... Anyway, please be aware that the MEMOP ioctl is defined as IOW only: #define KVM_S390_MEM_OP _IOW(KVMIO, 0xb1, struct kvm_s390_mem_op) ... so if you now introduce an "out" field in that struct, this might have some impact, e.g. on Valgrind etc. Thomas
diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst index abd7c32126ce..0e02d66e38ae 100644 --- a/Documentation/virt/kvm/api.rst +++ b/Documentation/virt/kvm/api.rst @@ -3771,6 +3771,7 @@ Parameters are specified via the following structure:: struct { __u8 ar; /* the access register number */ __u8 key; /* access key, ignored if flag unset */ + __u64 old[2]; /* ignored if flag unset */ }; __u32 sida_offset; /* offset into the sida */ __u8 reserved[32]; /* ignored */ @@ -3853,8 +3854,23 @@ 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 value provided in "old". This is performed +as an atomic cmpxchg. "size" must be a power of two up to and including 16. +Values with sizes <8 byte are to be provided by assignment to "old[1]". +Doublewords are provided with the higher value word in "old[0]" and the lower +word in "old[1]". +The value at the target location is returned in "old", encoded in the same manner. +If the value was exchanged the KVM_S390_MEMOP_F_CMPXCHG bit in "flags" is set to +0, otherwise it remains set. +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: ^^^^^^^^^^^^^^^^
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 | 18 +++++++++++++++++- 1 file changed, 17 insertions(+), 1 deletion(-)