Re: [PATCH v3 1/9] KVM: s390: Extend MEM_OP ioctl by storage key checked cmpxchg

Heiko Carstens
2022-11-18T15:15:28Z

On Fri, Nov 18, 2022 at 03:37:26PM +0100, Thomas Huth wrote:
> > > diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> > > index 0d5d4419139a..1f36be5493e6 100644
> > > --- a/include/uapi/linux/kvm.h
> > > +++ b/include/uapi/linux/kvm.h
> > > @@ -588,6 +588,8 @@ struct kvm_s390_mem_op {
> > > 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 */
> >
> > Just one comment: the suffix "_p" for pointer is quite unusual within
> > the kernel. This also would be the first of its kind within kvm.h.
> > Usually there is either no suffix or "_addr".
> > So for consistency reasons I would suggest to change this to one of
> > the common variants.
> >
> > The code itself looks good from my point of view, even though for the
> > sake of simplicity I would have put the complete sign/zero extended
> > 128 bit old value into the structure, instead of having a pointer to
> > the value.
>
> See
> https://lore.kernel.org/kvm/37197cfe-d109-332f-089b-266d7e8e23f8@redhat.com/
> ... it would break the "IOW" definition of the ioctl. It can be done, but
> that confuses tools like valgrind, as far as I know. So I think the idea
> with the pointer is better in this case.

Ah right, I forgot about that. Then let's do it this way.