Re: [PATCH v1 3/9] Documentation: KVM: s390: Describe KVM_S390_MEMOP_F_CMPXCHG

[Janis Schoetterl-Glausch]

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@xxxxxxxxxxxxx>
> > ---
> >   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
>