Re: [virtio-dev] [PATCH] virtio: support VIRTIO_F_ORDER_PLATFORM

[Jason Wang]

On 2019/1/23 äå11:49, Michael S. Tsirkin wrote:
> On Wed, Jan 23, 2019 at 11:08:04AM +0800, Jason Wang wrote:
> > On 2019/1/23 äå1:03, Tiwei Bie wrote:
> > > This patch introduces the support for VIRTIO_F_ORDER_PLATFORM.
> > > 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       | 8 ++++++++
> > >    include/uapi/linux/virtio_config.h | 6 ++++++
> > >    2 files changed, 14 insertions(+)
> > >
> > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > > index cd7e755484e3..27d3f057493e 100644
> > > --- a/drivers/virtio/virtio_ring.c
> > > +++ b/drivers/virtio/virtio_ring.c
> > > @@ -1609,6 +1609,9 @@ static struct virtqueue *vring_create_virtqueue_packed(
> > >    		!context;
> > >    	vq->event = virtio_has_feature(vdev, VIRTIO_RING_F_EVENT_IDX);
> > > +	if (virtio_has_feature(vdev, VIRTIO_F_ORDER_PLATFORM))
> > > +		vq->weak_barriers = false;
> > > +
> > >    	vq->packed.ring_dma_addr = ring_dma_addr;
> > >    	vq->packed.driver_event_dma_addr = driver_event_dma_addr;
> > >    	vq->packed.device_event_dma_addr = device_event_dma_addr;
> > > @@ -2079,6 +2082,9 @@ 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_ORDER_PLATFORM))
> > > +		vq->weak_barriers = false;
> > > +
> > >    	vq->split.queue_dma_addr = 0;
> > >    	vq->split.queue_size_in_bytes = 0;
> > > @@ -2213,6 +2219,8 @@ void vring_transport_features(struct virtio_device *vdev)
> > >    			break;
> > >    		case VIRTIO_F_RING_PACKED:
> > >    			break;
> > > +		case VIRTIO_F_ORDER_PLATFORM:
> > > +			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 1196e1c1d4f6..ff8e7dc9d4dd 100644
> > > --- a/include/uapi/linux/virtio_config.h
> > > +++ b/include/uapi/linux/virtio_config.h
> > > @@ -78,6 +78,12 @@
> > >    /* This feature indicates support for the packed virtqueue layout. */
> > >    #define VIRTIO_F_RING_PACKED		34
> > > +/*
> > > + * This feature indicates that memory accesses by the driver and the
> > > + * device are ordered in a way described by the platform.
> > > + */
> > > +#define VIRTIO_F_ORDER_PLATFORM		36
> > > +
> > >    /*
> > >     * Does the device support Single Root I/O Virtualization?
> > >     */
> >
> > I wonder whether or not this is sufficient. Is dma barrier implies a mmio
> > barrier? Looks not.
> IIUC we don't need an mmio barrier because we are using a
> serializing API: Documentation/memory-barriers.txt says:
>
> 	Note that, when using writel(), a prior
>       wmb() is not needed to guarantee that the cache coherent memory writes
>       have completed before writing to the MMIO region.


Ah, I get this.


> > See ia64/include/asm/barrier.h:
> >
> >  Â* Note: "mb()" and its variants cannot be used as a fence to order
> >  Â* accesses to memory mapped I/O registers. For that, mf.a needs to
> >  Â* be used. However, we don't want to always use mf.a because (a)
> >  Â* it's (presumably) much slower than mf and (b) mf.a is supported for
> >  Â* sequential memory pages only.
> >  Â*/
> > #define mb()ÂÂÂÂÂÂÂÂÂÂÂ ia64_mf()
> > #define rmb()ÂÂÂÂÂÂÂÂÂÂ mb()
> > #define wmb()ÂÂÂÂÂÂÂÂÂÂ mb()
> >
> > #define dma_rmb()ÂÂÂÂÂÂ mb()
> > =>efine dma_wmb()ÂÂÂÂÂÂ mb()
> >
> > Thanks
> Frankly no idea about ia64.


Neither did me.


>   Sorry. Are any less esoteric platforms
> affected?

E.g ppc64?

define dma_wmb()ÂÂÂÂÂÂ __asm__ __volatile__ (stringify_in_c(SMPWMB) : : 
:"memo\
ry")

/*
Â* Enforce synchronisation of stores vs. spin_unlock
Â* (this does it explicitly, though our implementation of spin_unlock
Â* does it implicitely too)
Â*/
static inline void mmiowb(void)
{
ÂÂÂÂÂÂÂ unsigned long tmp;

ÂÂÂÂÂÂÂ __asm__ __volatile__("sync; li %0,0; stb %0,%1(13)"
ÂÂÂÂÂÂÂ : "=&r" (tmp) : "i" (offsetof(struct paca_struct, io_sync))
ÂÂÂÂÂÂÂ : "memory");
}

dma_wmb() is lwsync which is more lightweight than sync I guess?

Thanks