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

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On 2019/1/23 上午11:49, Michael S. Tsirkin wrote:
On Wed, Jan 23, 2019 at 11:08:04AM +0800, Jason Wang wrote:
On 2019/1/23 上午1:03, 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:

  * Note: "mb()" and its variants cannot be used as a fence to order
  * accesses to memory mapped I/O registers.  For that, mf.a needs to
  * be used.  However, we don't want to always use mf.a because (a)
  * it's (presumably) much slower than mf and (b) mf.a is supported for
  * sequential memory pages only.
  */
#define mb()            ia64_mf()
#define rmb()           mb()
#define wmb()           mb()

#define dma_rmb()       mb()
=>efine dma_wmb()       mb()

Thanks
Frankly no idea about ia64.


Neither did me.


  Sorry. Are any less esoteric platforms
affected?


E.g ppc64?

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

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

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

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

Thanks


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