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

From: Thomas Huth
Date: Fri Nov 18 2022 - 09:41:54 EST


On 18/11/2022 11.12, Heiko Carstens wrote:
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@xxxxxxxxxxxxx>
---
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.

See https://lore.kernel.org/kvm/37197cfe-d109-332f-089b-266d7e8e23f8@xxxxxxxxxx/ ... 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.

Thomas