linux-kernel - Re: [PATCH] virtio-blk: set QUEUE_ORDERED

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Message-Id: <200908210930.39861.borntraeger@de.ibm.com>
Date:	Fri, 21 Aug 2009 09:30:39 +0200
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	Christoph Hellwig <hch@....de>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH] virtio-blk: set QUEUE_ORDERED_DRAIN by default

Am Donnerstag 20 August 2009 22:56:16 schrieb Christoph Hellwig:
> Currently virtio-blk doesn't set any QUEUE_ORDERED_ flag by default, which
> means it does not allow filesystems to use barriers.  But the typical use
> case for virtio-blk is to use a backed that uses synchronous I/O, and in
> that case we can simply set QUEUE_ORDERED_DRAIN to make the block layer
> drain the request queue around barrier I/O and provide the semantics that
> the filesystems need.  This is what the SCSI disk driver does for disks
> that have the write cache disabled.
>
> With this patch we incorrectly advertise barrier support if someone
> configure qemu with write back caching.  While this displays wrong
> information in the guest there is nothing that guest could have done
> even if we rightfully told it that we do not support any barriers.
>
> Signed-off-by: Christoph Hellwig <hch@....de>

Make sense to me.
Reviewed-by: Christian Borntraeger <borntraeger@...ibm.com>

[...]
> -	/* If barriers are supported, tell block layer that queue is ordered */
> +	/*
> +	 * If barriers are supported, tell block layer that queue is ordered.
> +	 *
> +	 * If no barriers are supported assume the host uses synchronous
> +	 * writes and just drain the the queue before and after the barrier.
> +	 */
>  	if (virtio_has_feature(vdev, VIRTIO_BLK_F_BARRIER))
>  		blk_queue_ordered(vblk->disk->queue, QUEUE_ORDERED_TAG, NULL);
> +	else
> +		blk_queue_ordered(vblk->disk->queue, QUEUE_ORDERED_DRAIN, NULL);
[...]
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