Re: [RFC] virtio: support VIRTIO_F_IO_BARRIER

[Tiwei Bie]

On Thu, May 03, 2018 at 03:30:03PM +0800, Jason Wang wrote:
> On 2018å05æ03æ 10:59, Tiwei Bie wrote:
> > This patch introduces the support for VIRTIO_F_IO_BARRIER.
> > When this feature is negotiated, driver will use the barriers
> > suitable for hardware devices.
> > 
> > Signed-off-by: Tiwei Bie <tiwei.bie@xxxxxxxxx>
> > ---
> >   drivers/virtio/virtio_ring.c       | 5 +++++
> >   include/uapi/linux/virtio_config.h | 8 +++++++-
> >   2 files changed, 12 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index 21d464a29cf8..edb565643bf4 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -996,6 +996,9 @@ struct virtqueue *__vring_new_virtqueue(unsigned int index,
> >   		!context;
> >   	vq->event = virtio_has_feature(vdev, VIRTIO_RING_F_EVENT_IDX);
> > +	if (virtio_has_feature(vdev, VIRTIO_F_IO_BARRIER))
> > +		vq->weak_barriers = false;
> > +
> >   	/* No callback?  Tell other side not to bother us. */
> >   	if (!callback) {
> >   		vq->avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
> > @@ -1164,6 +1167,8 @@ void vring_transport_features(struct virtio_device *vdev)
> >   			break;
> >   		case VIRTIO_F_IOMMU_PLATFORM:
> >   			break;
> > +		case VIRTIO_F_IO_BARRIER:
> > +			break;
> >   		default:
> >   			/* We don't understand this bit. */
> >   			__virtio_clear_bit(vdev, i);
> > diff --git a/include/uapi/linux/virtio_config.h b/include/uapi/linux/virtio_config.h
> > index 308e2096291f..6ca8d24bf468 100644
> > --- a/include/uapi/linux/virtio_config.h
> > +++ b/include/uapi/linux/virtio_config.h
> > @@ -49,7 +49,7 @@
> >    * transport being used (eg. virtio_ring), the rest are per-device feature
> >    * bits. */
> >   #define VIRTIO_TRANSPORT_F_START	28
> > -#define VIRTIO_TRANSPORT_F_END		34
> > +#define VIRTIO_TRANSPORT_F_END		38
> >   #ifndef VIRTIO_CONFIG_NO_LEGACY
> >   /* Do we get callbacks when the ring is completely used, even if we've
> > @@ -71,4 +71,10 @@
> >    * this is for compatibility with legacy systems.
> >    */
> >   #define VIRTIO_F_IOMMU_PLATFORM		33
> > +
> > +/*
> > + * If clear - driver may use barriers suitable for CPU cores.
> > + * If set - driver must use barriers suitable for hardware devices.
> > + */
> > +#define VIRTIO_F_IO_BARRIER		37
> >   #endif /* _UAPI_LINUX_VIRTIO_CONFIG_H */
> 
> Hi:
> 
> I believe this depends on Michael's patch of
> 
> "[PATCH] virtio_ring: switch to dma_XX barriers for rpmsg"
> 
> ?
> 
> Thanks

We already have below commit and some other related commits
in the tree:

7b21e34fd1c2 ("virtio: harsher barriers for rpmsg.")

They should have already guaranteed that virtio_Xmb() will
be OK for hardware devices when vq->weak_barriers is false.
If my understanding is correct, the barriers used in this
case are overkill. So Michael's patch is to make the barriers
weaker (or better).

Best regards,
Tiwei Bie