From mboxrd@z Thu Jan  1 00:00:00 1970
From: Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH V3 3/3] virtio-blk: Add bio-based IO path for virtio-blk
Date: Sat, 28 Jul 2012 02:42:33 -0400 (EDT)
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To: Asias He <asias@redhat.com>
Cc: kvm@vger.kernel.org, "Michael S. Tsirkin" <mst@redhat.com>, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, Christoph Hellwig <hch@lst.de>
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> > I'm not sure what the correct behavior for bio & cacheflush is, if
> > any.
> 
> REQ_FLUSH is not supported in the bio path.

Ouch, that's correct:

@@ -414,7 +529,7 @@ static void virtblk_update_cache_mode(struct virtio_device *vdev)
 	u8 writeback = virtblk_get_cache_mode(vdev);
 	struct virtio_blk *vblk = vdev->priv;
 
-	if (writeback)
+	if (writeback && !use_bio)
 		blk_queue_flush(vblk->disk->queue, REQ_FLUSH);
 	else
 		blk_queue_flush(vblk->disk->queue, 0);

then it is not safe against power losses.

Paolo