[virtio-comment] virtio-sound linux driver conformance to spec

Paolo Bonzini
Wed, 13 Sep 2023 15:50:48 -0000

On Wed, Sep 13, 2023 at 5:05?PM Matias Ezequiel Vara Larsen
<mvaralar at redhat.com> wrote:
>
> Hello,
>
> This email is to report a behavior of the Linux virtio-sound driver that
> looks like it is not conforming to the VirtIO specification. The kernel
> driver is moving buffers from the used ring to the available ring
> without knowing if the content has been updated from the user. If the
> device picks up buffers from the available ring just after it is
> notified, it happens that the content is old. This problem can be fixed
> by waiting a period of time after the device dequeues a buffer from the
> available ring. The driver should not be allowed to change the content
> of a buffer in the available ring. When buffers are enqueued in the
> available ring, the device can consume them immediately.
>
> 1. Is the actual driver implementation following the spec?

If I understand the issue correctly, it's not. As you say, absent any
clarification a buffer that has been placed in the available ring
should be unmodifiable; the driver should operate as if the data in
the available ring is copied to an internal buffer instantly when the
buffer id is added to the ring.

I am assuming this is a playback buffer. To clarify, does the driver
expect buffers to be read only as needed, which is a fraction of a
second before the data is played back?

> 2. Shall the spec be extended to support such a use-case?

If it places N buffers, I think it's a reasonable expectation (for the
sound device only!) that the Nth isn't read until the (N-1)th has
started playing. With two buffers only, the behavior you specify would
not be permissible (because as soon as buffer 1 starts playing, the
device can read buffer 2; there is never an idle buffer). With three
buffers, you could write buffer 3 while buffer 1 plays; write buffer 1
while buffer 2 plays; and write buffer 2 while buffer 3 plays. Is this
enough?

That said, being reasonable isn't enough for virtio-sound to do it and
deviate from other virtio devices. Why does the Linux driver behave
like this? Is it somehow constrained by the kernel->userspace APIs?

Paolo