Re: [PATCH vhost v4 1/6] virtio_balloon: remove the dependence where names[] is null

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On Mon, Mar 25 2024, Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx> wrote:

> On Fri, 22 Mar 2024 22:02:27 +0100, David Hildenbrand <david@xxxxxxxxxx> wrote:
>> On 22.03.24 20:16, Daniel Verkamp wrote:
>> > On Thu, Mar 21, 2024 at 3:16 AM Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx> wrote:
>> >>
>> >> Currently, the init_vqs function within the virtio_balloon driver relies
>> >> on the condition that certain names array entries are null in order to
>> >> skip the initialization of some virtual queues (vqs). This behavior is
>> >> unique to this part of the codebase. In an upcoming commit, we plan to
>> >> eliminate this dependency by removing the function entirely. Therefore,
>> >> with this change, we are ensuring that the virtio_balloon no longer
>> >> depends on the aforementioned function.
>> >
>> > This is a behavior change, and I believe means that the driver no
>> > longer follows the spec [1].
>> >
>> > For example, the spec says that virtqueue 4 is reporting_vq, and
>> > reporting_vq only exists if VIRTIO_BALLOON_F_PAGE_REPORTING is set,
>> > but there is no mention of its virtqueue number changing if other
>> > features are not set. If a device/driver combination negotiates
>> > VIRTIO_BALLOON_F_FREE_PAGE_HINT, my reading of the specification is
>> > that reporting_vq should still be vq number 4, and vq 2 and 3 should
>> > be unused. This patch would make the reporting_vq use vq 2 instead in
>> > this case.
>> >
>> > If the new behavior is truly intended, then the spec does not match
>> > reality, and it would need to be changed first (IMO); however,
>> > changing the spec would mean that any devices implemented correctly
>> > per the previous spec would now be wrong, so some kind of mechanism
>> > for detecting the new behavior would be warranted, e.g. a new
>> > non-device-specific virtio feature flag.
>> >
>> > I have brought this up previously on the virtio-comment list [2], but
>> > it did not receive any satisfying answers at that time.

I had missed it back then, but now that I read it, I realize that we
really have a bit of a mess here :/

>> Rings a bell, but staring at this patch, I thought that there would be
>> no behavioral change. Maybe I missed it :/
>> I stared at virtio_ccw_find_vqs(), and it contains:
>> 	for (i = 0; i < nvqs; ++i) {
>> 		if (!names[i]) {
>> 			vqs[i] = NULL;
>> 			continue;
>> 		}
>> 		vqs[i] = virtio_ccw_setup_vq(vdev, queue_idx++, callbacks[i],
>> 					     names[i], ctx ? ctx[i] : false,
>> 					     ccw);
>> 		if (IS_ERR(vqs[i])) {
>> 			ret = PTR_ERR(vqs[i]);
>> 			vqs[i] = NULL;
>> 			goto out;
>> 		}
>> 	}
>> We increment queue_idx only if an entry was not NULL. SO I thought no
>> behavioral change? (at least on s390x :) )

The code for pci behaves in the same way.

>> It's late here in Germany, so maybe I'm missing something.
> I think we've encountered a tricky issue. Currently, all transports handle queue
> id by incrementing them in order, without skipping any queue id. So, I'm quite
> surprised that my changes would affect the spec. The fact that the
> 'names' value is null is just a small trick in the Linux kernel implementation
> and should not have an impact on the queue id.
> I believe that my recent modification will not affect the spec. So, let's
> consider the issues with this patch set separately for now. Regarding the Memory
> Balloon Device, it has been operational for many years, and perhaps we should
> add to the spec that if a certain vq does not exist, then subsequent vqs will
> take over its id.

The changes here do not really seem to affect the spec issue that Daniel
had noted, unless I'm reading the code wrong.

However, we should try to address the spec mess, where we have at least
some of the most popular/important implementations behaving differently
than the spec describes... I would suggest to discuss that on the virtio
lists -- but they are still dead, and at this point I'm just hoping
they'll come back eventually :/

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