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@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.
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.
Thomas