Re: [PATCH v5 01/10] KVM: s390: Extend MEM_OP ioctl by storage key checked cmpxchg

[Thomas Huth <thuth@redhat.com>]

On 11/01/2023 11.00, Janis Schoetterl-Glausch wrote:
> On Wed, 2023-01-11 at 08:59 +0100, Thomas Huth wrote:
>> On 10/01/2023 21.26, 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 |   7 +++
>>>    arch/s390/kvm/gaccess.h  |   3 ++
>>>    arch/s390/kvm/gaccess.c  | 102 +++++++++++++++++++++++++++++++++++++++
>>>    arch/s390/kvm/kvm-s390.c |  41 +++++++++++++++-
>>>    4 files changed, 151 insertions(+), 2 deletions(-)
>>>
> [...]
> 
>>> +/**
>>> + * cmpxchg_guest_abs_with_key() - Perform cmpxchg on guest absolute address.
>>> + * @kvm: Virtual machine instance.
>>> + * @gpa: Absolute guest address of the location to be changed.
>>> + * @len: Operand length of the cmpxchg, required: 1 <= len <= 16. Providing a
>>> + *       non power of two will result in failure.
>>> + * @old_addr: Pointer to old value. If the location at @gpa contains this value, the
>>> + *         exchange will succeed. After calling cmpxchg_guest_abs_with_key() *@old
>>> + *         contains the value at @gpa before the attempt to exchange the value.
>>> + * @new: The value to place at @gpa.
>>> + * @access_key: The access key to use for the guest access.
>>> + *
>>> + * Atomically exchange the value at @gpa by @new, if it contains *@old.
>>> + * Honors storage keys.
>>> + *
>>> + * Return: * 0: successful exchange
>>> + *         * 1: exchange unsuccessful
>>> + *         * a program interruption code indicating the reason cmpxchg could
>>> + *           not be attempted
>>
>> PGM_OPERATION has also the value 1 ... can we be sure that it never happens
>> here?
> 
> Currently yes, only program errors are those explicit in the code,
> PGM_ADDRESSING and PGM_PROTECTION.
> 
>> ... maybe it would make sense to use KVM_S390_MEMOP_R_NO_XCHG for
>> return value here instead of 1, too, just to be on the safe side?
> 
> I didn't like that idea because I consider KVM_S390_MEMOP_R_NO_XCHG to be
> part of the KVM's api surface and cmpxchg_guest_abs_with_key is an internal
> function that shouldn't concern itself with that.
> 
> But being unclear on PGM_OPERATION is indeed ugly.
> Maybe I should just replace "a program interruption code ..." with the specific ones?

Yes, that would help to avoid this confusion. With such a change feel free 
to add:
Reviewed-by: Thomas Huth <thuth@redhat.com>