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 <[email protected]>
---
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 */
+ __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:
^^^^^^^^^^^^^^^^
--
2.34.1
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 <[email protected]>
> ---
> 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 <[email protected]>
> > ---
> > 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 <[email protected]>
>>> ---
>>> 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