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Date: Wed, 1 Aug 2012 11:31:29 +0100
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From: Stefan Hajnoczi <stefanha@gmail.com>
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Subject: Re: [Qemu-devel] virtio-blk: Something bizarre with
	VIRTIO_BLK_T_GET_ID
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To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Anthony Liguori <aliguori@us.ibm.com>, qemu-devel@nongnu.org

On Wed, Aug 1, 2012 at 11:27 AM, Benjamin Herrenschmidt
<benh@kernel.crashing.org> wrote:
> On Wed, 2012-08-01 at 11:16 +0100, Stefan Hajnoczi wrote:
>
>> The len field is mostly informational.  The virtio device driver
>> inside the guest may make use of it.  In many cases it doesn't so an
>> incorrect len value has no effect.  In
>> drivers/block/virtio_blk.c:blk_done() the len variable is unused.
>
> Right but arguably the guest reading more than the len passed back into
> the descriptor is itself a bug :-)
>
>> QEMU should call cpu_physical_memory_unmap() with the correct size
>> value in hw/virtio.c:virtqueue_fill() so that the memory dirty bitmap
>> is kept up-to-date.  This is the only bad side-effect I can see here.
>
> With the current guest driver ... another one adhering strictly to the
> spec might get bitten :-) Anyway, it's minor, but probably somebody
> should fix. I don't have time right now, but if you don't beat me to it
> I might try to give it a spin tomorrow.

Added to my TODO list but I probably won't get a chance before
tomorrow either because I need to focus on QEMU 1.2 soft freeze.

Stefan