linux-kernel - Re: [virtio-dev] [PATCH] virtio: support VIRTIO_F_ORDER

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Date:   Wed, 23 Jan 2019 11:08:04 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Tiwei Bie <tiwei.bie@...el.com>, mst@...hat.com,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, virtio-dev@...ts.oasis-open.org
Subject: Re: [virtio-dev] [PATCH] virtio: support VIRTIO_F_ORDER_PLATFORM


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@...el.com>
> ---
>   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.

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