On Fri, Oct 27, 2023 at 12:18:00PM +0200, Stefano Garzarella wrote: > On Fri, Oct 27, 2023 at 11:27:40AM +0200, Takashi Iwai wrote: > > On Wed, 25 Oct 2023 11:49:19 +0200, > > Matias Ezequiel Vara Larsen wrote: > > > > > > This commit uses the ack() callback to determine when a buffer has been > > > updated, then exposes it to guest. > > > > > > The current mechanism splits a dma buffer into descriptors that are > > > exposed to the device. This dma buffer is shared with the user > > > application. When the device consumes a buffer, the driver moves the > > > request from the used ring to available ring. > > > > > > The driver exposes the buffer to the device without knowing if the > > > content has been updated from the user. The section 2.8.21.1 of the > > > virtio spec states that: "The device MAY access the descriptor chains > > > the driver created and the memory they refer to immediately". If the > > > device picks up buffers from the available ring just after it is > > > notified, it happens that the content may be old. > > > > > > When the ack() callback is invoked, the driver exposes only the buffers > > > that have already been updated, i.e., enqueued in the available ring. > > > Thus, the device always picks up a buffer that is updated. > > > > > > For capturing, the driver starts by exposing all the available buffers > > > to device. After device updates the content of a buffer, it enqueues it > > > in the used ring. It is only after the ack() for capturing is issued > > > that the driver re-enqueues the buffer in the available ring. > > > > > > Co-developed-by: Anton Yakovlev <anton.yakovlev@xxxxxxxxxxxxxxx> > > > Signed-off-by: Anton Yakovlev <anton.yakovlev@xxxxxxxxxxxxxxx> > > > Signed-off-by: Matias Ezequiel Vara Larsen <mvaralar@xxxxxxxxxx> > > > > Applied now to for-next branch. > > Cool, thanks for that! > > I just wonder if we should CC stable since we are fixing a virtio > specification violation. > > @Michael what do you think? > > Thanks, > Stefano Acked-by: Michael S. Tsirkin <mst@xxxxxxxxxx> Fixes: de3a9980d8c3 ("ALSA: virtio: add virtio sound driver") The patch is too big for stable - more than 100 lines added. See: Documentation/process/stable-kernel-rules.rst -- MST