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

[Heiko Carstens <hca@linux.ibm.com>]

On Thu, Nov 17, 2022 at 11:17:50PM +0100, Janis Schoetterl-Glausch wrote:
> User space can use the MEM_OP ioctl to make storage key checked reads
> and writes to the guest, however, it has no way of performing atomic,
> key checked, accesses to the guest.
> Extend the MEM_OP ioctl in order to allow for this, by adding a cmpxchg
> mode. For now, support this mode for absolute accesses only.
> 
> This mode can be use, for example, to set the device-state-change
> indicator and the adapter-local-summary indicator atomically.
> 
> Signed-off-by: Janis Schoetterl-Glausch <scgl@linux.ibm.com>
> ---
>  include/uapi/linux/kvm.h |   5 ++
>  arch/s390/kvm/gaccess.h  |   3 ++
>  arch/s390/kvm/gaccess.c  | 101 +++++++++++++++++++++++++++++++++++++++
>  arch/s390/kvm/kvm-s390.c |  35 +++++++++++++-
>  4 files changed, 142 insertions(+), 2 deletions(-)
> 
> 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. Imho that would simplify the interface. Also alignment, as
pointed out previously, really doesn't matter for this use case.

But you had already something like that previously and changed it, so
no reason to go back and forth. Not really important.