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. > > MatiasSounds 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
Matias Ezequiel Vara Larsen
2023-Sep-20 13:18 UTC
virtio-sound linux driver conformance to spec
On Tue, Sep 19, 2023 at 11:52:27AM -0400, Michael S. Tsirkin wrote:> 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?AFAIU, a fix for the driver may be to somehow wait until userspace updates the buffer before add it in the available ring. So far, when the device notifies the driver that a new buffer is in the used ring, the driver calls the virtsnd_pcm_msg_complete() function to do: ``` schedule_work(&vss->elapsed_period); virtsnd_pcm_msg_send(vss); ``` It first defers the notification to userspace regarding an elapse period and then it enqueues the request again in the available ring.`schedule_work()` defers the calling to the `virtsnd_pcm_period_elapsed()` function that issues `snd_pcm_period_elapsed(vss->substream);` to notify userspace. My proposal would be that the driver could also defer `virtsnd_pcm_msg_send(vss)` to execute just after `snd_pcm_period_elapsed(vss->substream)`. Something like this: diff --git a/sound/virtio/virtio_pcm.c b/sound/virtio/virtio_pcm.c index c10d91fff2fb..41f1e74c8478 100644 --- a/sound/virtio/virtio_pcm.c +++ b/sound/virtio/virtio_pcm.c @@ -309,6 +309,7 @@ static void virtsnd_pcm_period_elapsed(struct work_struct *work) container_of(work, struct virtio_pcm_substream, elapsed_period); snd_pcm_period_elapsed(vss->substream); + virtsnd_pcm_msg_send(vss); } /** diff --git a/sound/virtio/virtio_pcm_msg.c b/sound/virtio/virtio_pcm_msg.c index aca2dc1989ba..43f0078b1152 100644 --- a/sound/virtio/virtio_pcm_msg.c +++ b/sound/virtio/virtio_pcm_msg.c @@ -321,7 +321,6 @@ static void virtsnd_pcm_msg_complete(struct virtio_pcm_msg *msg, schedule_work(&vss->elapsed_period); - virtsnd_pcm_msg_send(vss); } else if (!vss->msg_count) { wake_up_all(&vss->msg_empty); } I tested it and it looks it fixes the issue. However, I am not sure if this is enough since I do not know if when `snd_pcm_period_elapsed()` returns, the buffers have been already updated. Matias
Hello Michael, On 20.09.2023 00:52, Michael S. Tsirkin wrote:> 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...Fixing the problem Matias described only requires changes to the driver. But we will need to restrict applications from mmap()'ing the buffer. Applications will be able to read/write frames only through ioctl() requests. I think we could expand the sound specification to add support for shared memory. Then it should be possible to implement mmap() support on top of it. -- Anton Yakovlev Senior Software Engineer OpenSynergy GmbH Rotherstr. 20, 10245 Berlin