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

[Stefan Hajnoczi <stefanha@redhat.com>]

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 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 QEMU's VirtQueueElement? There are devices in QEMU that
save/load in-flight VirtQueueElements. See qemu_put_virtqueue_element().

Often devices quiesce (e.g. by draining in-flight I/O requests) when the
VM is stopped before the device state is saved. That makes life simpler.

Stefan

> 
> Thank you.
> 
> Regards,
> Roman.
> 
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