On Sat, 2025-11-15 at 12:08 +0200, Manos Pitsidianakis wrote:
> On Fri, Nov 14, 2025 at 11:35 AM Yanfeng Liu <[email protected]> wrote:
> >
> > Media players needs meaningful latency_bytes update but it is
> > zero now most of the time. This adds stream-wise latency_bytes
> > calculation so that to improve the situation.
> >
> > Signed-off-by: Yanfeng Liu <[email protected]>
> > ---
>
> Thanks for this patch. The virtio spec doesn't adequately explain what
> latency_bytes means, could you explain what it does and why it's
Thanks for your attention!
Yes the spec is not clear. I saw issue 198 in github.com/oasis-tcs/virtio-spec
after seeing your question. I think that discussion may clarify this on spec
later.
> needed? And also what behavior in media players this patch fixes?
The field helps guest players to adjust their data feeding operations less
blindly and improve system efficiency, especially when multiple streams are
used. Here we noticed it when came from the goldfish emulator, so we are
wondering if sharing this can be of help.
>
> Thanks again!
>
> > hw/audio/virtio-snd.c | 12 +++++++++++-
> > include/hw/audio/virtio-snd.h | 1 +
> > 2 files changed, 12 insertions(+), 1 deletion(-)
> >
> > diff --git a/hw/audio/virtio-snd.c b/hw/audio/virtio-snd.c
> > index 9101560f38..ed0422b45a 100644
> > --- a/hw/audio/virtio-snd.c
> > +++ b/hw/audio/virtio-snd.c
> > @@ -431,6 +431,7 @@ static uint32_t virtio_snd_pcm_prepare(VirtIOSound *s,
> > uint32_t stream_id)
> > stream->id = stream_id;
> > stream->pcm = s->pcm;
> > stream->s = s;
> > + stream->latency_bytes = 0;
> > qemu_mutex_init(&stream->queue_mutex);
> > QSIMPLEQ_INIT(&stream->queue);
> >
> > @@ -899,6 +900,7 @@ static void virtio_snd_handle_tx_xfer(VirtIODevice
> > *vdev, VirtQueue *vq)
> > buffer->vq = vq;
> > buffer->size = size;
> > buffer->offset = 0;
> > + stream->latency_bytes += size;
> >
> > QSIMPLEQ_INSERT_TAIL(&stream->queue, buffer, entry);
> > }
> > @@ -1112,12 +1114,19 @@ error_cleanup:
> > virtio_snd_unrealize(dev);
> > }
> >
> > +static inline void update_latency(VirtIOSoundPCMStream *s, size_t used)
> > +{
> > + s->latency_bytes = s->latency_bytes > used ?
> > + s->latency_bytes - used : 0;
> > +}
> > +
> > static inline void return_tx_buffer(VirtIOSoundPCMStream *stream,
> > VirtIOSoundPCMBuffer *buffer)
> > {
> > virtio_snd_pcm_status resp = { 0 };
> > resp.status = cpu_to_le32(VIRTIO_SND_S_OK);
> > - resp.latency_bytes = cpu_to_le32((uint32_t)buffer->size);
> > + update_latency(stream, buffer->size);
> > + resp.latency_bytes = cpu_to_le32(stream->latency_bytes);
> > iov_from_buf(buffer->elem->in_sg,
> > buffer->elem->in_num,
> > 0,
> > @@ -1178,6 +1187,7 @@ static void virtio_snd_pcm_out_cb(void *data, int
> > available)
> > buffer->size -= size;
> > buffer->offset += size;
> > available -= size;
> > + update_latency(stream, size);
> > if (buffer->size < 1) {
> > return_tx_buffer(stream, buffer);
> > break;
> > diff --git a/include/hw/audio/virtio-snd.h b/include/hw/audio/virtio-snd.h
> > index c176066584..9560bac8b1 100644
> > --- a/include/hw/audio/virtio-snd.h
> > +++ b/include/hw/audio/virtio-snd.h
> > @@ -150,6 +150,7 @@ struct VirtIOSoundPCMStream {
> > } voice;
> > QemuMutex queue_mutex;
> > bool active;
> > + uint32_t latency_bytes;
> > QSIMPLEQ_HEAD(, VirtIOSoundPCMBuffer) queue;
> > };
> >
> > --
> > 2.34.1
