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

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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.

-- Daniel


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