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_PAGE_REPORTING but not VIRTIO_BALLOON_F_STATS_VQ or
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;
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 :) )
It's late here in Germany, so maybe I'm missing something.
--
Cheers,
David / dhildenb
