linux-kernel - Re: [RFC PATCH 1/2] include/uapi/linux/virtio_blk.h: introduce feature of VIRTIO_BLK_F

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Date:	Mon, 16 Jun 2014 22:12:50 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Ming Lei <ming.lei@...onical.com>, Jens Axboe <axboe@...nel.dk>,
	linux-kernel@...r.kernel.org
Cc:	linux-api@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Stefan Hajnoczi <stefanha@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Ming Lei <ming.lei@...onical.com>
Subject: Re: [RFC PATCH 1/2] include/uapi/linux/virtio_blk.h: introduce feature of VIRTIO_BLK_F_MQ

Ming Lei <ming.lei@...onical.com> writes:
> Current virtio-blk spec only supports one virtual queue for transfering
> data between VM and host, and inside VM all kinds of operations on
> the virtual queue needs to hold one lock, so cause below problems:
>
> 	- no scalability
>     - bad throughput
>
> So this patch requests to introduce feature of VIRTIO_BLK_F_MQ
> so that more than one virtual queues can be used to virtio-blk
> device, then above problems can be solved or eased.
>
> Signed-off-by: Ming Lei <ming.lei@...onical.com>
> ---
>  include/uapi/linux/virtio_blk.h |    4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/include/uapi/linux/virtio_blk.h b/include/uapi/linux/virtio_blk.h
> index 6d8e61c..c5a2751 100644
> --- a/include/uapi/linux/virtio_blk.h
> +++ b/include/uapi/linux/virtio_blk.h
> @@ -40,6 +40,7 @@
>  #define VIRTIO_BLK_F_WCE	9	/* Writeback mode enabled after reset */
>  #define VIRTIO_BLK_F_TOPOLOGY	10	/* Topology information is available */
>  #define VIRTIO_BLK_F_CONFIG_WCE	11	/* Writeback mode available in config */
> +#define VIRTIO_BLK_F_MQ		12	/* support more than one vq */
>  
>  #ifndef __KERNEL__
>  /* Old (deprecated) name for VIRTIO_BLK_F_WCE. */
> @@ -77,6 +78,9 @@ struct virtio_blk_config {
>  
>  	/* writeback mode (if VIRTIO_BLK_F_CONFIG_WCE) */
>  	__u8 wce;
> +
> +	/* number of vqs, only available when VIRTIO_BLK_F_MQ is set */
> +	__u16 num_queues;
>  } __attribute__((packed));

Hmm, please pad this like so:

        __u8 unused;

        __u16 num_queues;

That avoids weird alignment.

Thanks,
Rusty.
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