Re: [virtio-dev] virtio-snd and snapshots (e.g. in QEMU) when audio is active

[Matias Ezequiel Vara Larsen <mvaralar@redhat.com>]

Hello Roman,

On Wed, Jul 19, 2023 at 04:21:08PM -0700, Roman Kiryanov wrote:
> Hi,
> 
> I work in Android Studio Emulator and we use virtio-snd (implemented
> ourselves) for audio output/input. According to the spec (1.2), the
> device has one TX virtqueue for all output streams and one RX
> virtqueue for all input streams. Each stream may and usually have more
> than one period (I request 4 periods).
> 
> Because virtqueues are shared between streams (if there are more than
> one stream in the same direction), I cannot fetch vq messages when a
> stream needs one. I fetch vq messages (and put them into my own buffer
> to process them later) when the kernel puts them into a vq. I hope
> this is correct. 

I think this is correct. If I understand correctly, this is the idea of
the pre-buffering after the PREPARE command, i.e., to feed the audio
engine before to START streaming (see section 5.14.6.6.1 PCM Command
Lifecycle).

> I think I tried processing them immediately (at least
> for TX) but the kernel was not happy with this because I was draining
> the buffer too fast causing XRUN.
> If a snapshot request comes when audio streams are active I may have
> several unprocessed messages for several streams for both TX and RX.
> In my case messages are VirtQueueElement* which I don't think can be
> saved directly.
> 
> Could you please advise what a device is expected to do in this case?
> 

Do you mean what a VMM shall do when taking a snapshot regarding the
virtio-snd device?

Matias