Re: [PATCH v1] s390/virtio_ccw: don't allocate/assign airqs for non-existing queues

[David Hildenbrand <david@redhat.com>]

On 09.04.25 14:07, Michael S. Tsirkin wrote:
> On Wed, Apr 09, 2025 at 01:12:19PM +0200, David Hildenbrand wrote:
>> On 09.04.25 12:56, Michael S. Tsirkin wrote:
>>> On Wed, Apr 09, 2025 at 12:46:41PM +0200, David Hildenbrand wrote:
>>>> On 07.04.25 23:20, Michael S. Tsirkin wrote:
>>>>> On Mon, Apr 07, 2025 at 08:47:05PM +0200, David Hildenbrand wrote:
>>>>>>> In my opinion, it makes the most sense to keep the spec as it is and
>>>>>>> change QEMU and the kernel to match, but obviously that's not trivial
>>>>>>> to do in a way that doesn't break existing devices and drivers.
>>>>>>
>>>>>> If only it would be limited to QEMU and Linux ... :)
>>>>>>
>>>>>> Out of curiosity, assuming we'd make the spec match the current QEMU/Linux
>>>>>> implementation at least for the 3 involved features only, would there be a
>>>>>> way to adjust crossvm without any disruption?
>>>>>>
>>>>>> I still have the feeling that it will be rather hard to get that all
>>>>>> implementations match the spec ... For new features+queues it will be easy
>>>>>> to force the usage of fixed virtqueue numbers, but for free-page-hinting and
>>>>>> reporting, it's a mess :(
>>>>>
>>>>>
>>>>> Still thinking about a way to fix drivers... We can discuss this
>>>>> theoretically, maybe?
>>>>
>>>> Yes, absolutely. I took the time to do some more digging; regarding drivers
>>>> only Linux seems to be problematic.
>>>>
>>>> virtio-win, FreeBSD, NetBSD and OpenBSD and don't seem to support
>>>> problematic features (free page hinting, free page reporting) in their
>>>> virtio-balloon implementations.
>>>>
>>>> So from the known drivers, only Linux is applicable.
>>>>
>>>> reporting_vq is either at idx 4/3/2
>>>> free_page_vq is either at idx 3/2
>>>> statsq is at idx2 (only relevant if the feature is offered)
>>>>
>>>> So if we could test for the existence of a virtqueue at an idx easily, we
>>>> could test from highest-to-smallest idx.
>>>>
>>>> But I recall that testing for the existance of a virtqueue on s390x resulted
>>>> in the problem/deadlock in the first place ...
>>>>
>>>> -- 
>>>> Cheers,
>>>>
>>>> David / dhildenb
>>>
>>> So let's talk about a new feature bit?
>>
>> Are you thinking about a new feature that switches between "fixed queue
>> indices" and "compressed queue indices", whereby the latter would be the
>> legacy default and we would expect all devices to switch to the new
>> fixed-queue-indices layout?
>>
>> We could make all new features require "fixed-queue-indices".
> 
> I see two ways:
> 1. we make driver behave correctly with in spec and out of spec devices
>     and we make qemu behave correctly with in spec and out of spec devices
> 2. a new feature bit
> 
> I prefer 1, and when we add a new feature we can also
> document that it should be in spec if negotiated.
> 
> My question is if 1 is practical.

AFAIKT, 1) implies:

virtio-balloon:

a) Driver

As mentioned above, we'd need a reliable way to test for the existence 
of a virtqueue, so we can e.g., test for reporting_vq idx 4 -> 3 -> 2

With that we'd be able to support compressed+fixed at the same time.

Q: Is it possible/feasible?

b) Device: virtio-balloon: we can fake existence of STAT and 
FREE_PAGE_HINTING easily, such that the compressed layout corresponds to 
the fixed layout easily.

Q: alternatives? We could try creating multiple queues for the same 
feature, but it's going to be a mess I'm afraid ...


virtio-fs:

a) Driver

Linux does not even implement VIRTIO_FS_F_NOTIFICATION or respect 
VIRTIO_FS_F_NOTIFICATION when calculating queue indices, ...

b) Device

Same applies to virtiofsd ...

Q: Did anybody actually implement VIRTIO_FS_F_NOTIFICATION ever? If not, 
can we just remove it from the spec completely and resolve the issue 
that way?

-- 
Cheers,

David / dhildenb