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

[Thomas Huth <thuth@redhat.com>]

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