Re: [RFC] virtio: support VIRTIO_F_IO_BARRIER

[Tiwei Bie]

On Thu, May 03, 2018 at 05:09:44PM +0800, Jason Wang wrote:
> On 2018å05æ03æ 16:30, Tiwei Bie wrote:
> > 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
> 
> Well, I think we need dma barriers for some platforms according to previous
> discussion? Without Michael's patch, we won't use any dma barriers in fact
> for virtio.

You are right. Thanks! Are you suggesting to add a
reference to Michael's patch in this patch to make
sure that it won't be applied before that patch?

Best regards,
Tiwei Bie

> 
> Thanks