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

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On 25.03.24 07:08, Xuan Zhuo 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.

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;

		vqs[i] = virtio_ccw_setup_vq(vdev, queue_idx++, callbacks[i],
					     names[i], ctx ? ctx[i] : false,
		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 :) )

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.

Right, if I am not missing something your patch should have no functional change in that regard (that the current behavior/implementation might not match the spec is a different discussion).

@Daniel, if I'm missing something, please shout.


David / dhildenb

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