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From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Date: Wed, 01 Aug 2012 20:27:52 +1000
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Subject: Re: [Qemu-devel] virtio-blk: Something bizarre with
 VIRTIO_BLK_T_GET_ID
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: Anthony Liguori <aliguori@us.ibm.com>, qemu-devel@nongnu.org

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.

Cheers,
Ben.