Re: virtio-sound linux driver conformance to spec

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Sep 19, 2023 at 04:18:57PM +0200, Matias Ezequiel Vara Larsen wrote:
> On Tue, Sep 19, 2023 at 05:43:56AM -0400, Michael S. Tsirkin wrote:
> > On Wed, Sep 13, 2023 at 05:04:24PM +0200, Matias Ezequiel Vara Larsen 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.
> > 
> > Then, what happens, exactly? Do things still work?
> 
> We are currently developing a vhost-user backend for virtio-sound and
> what happens is that if the backend implementation decides to copy the
> content of a buffer from a request that just arrived to the available
> ring, it gets the old content thus reproducing some sections two times.
> For example, we observe that when issuing `aplay FrontLeft.wav`, we hear
> `Front, front left...`. To fix this issue, our current implementation
> delays reading from guest memory just until the audio engine requires.
> However, the first implementation shall also work since it is conforming
> to the specification.
> 
> Matias

Sounds like it. How hard is it to change the behaviour though?
Does it involve changing userspace?
Maybe we need to fix the spec after all...

-- 
MST

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