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

On Fri, Nov 18, 2022 at 03:37:26PM +0100, Thomas Huth wrote:

quoted

quoted

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/ (local)
... 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.