Re: [Qemu-devel] [qemu-devel]question on virtqueue_get_avail_bytes

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Sep 03, 2013 at 04:40:21PM +0530, Amit Shah wrote:
> On (Mon) 19 Aug 2013 [16:30:54], Stefan Hajnoczi wrote:
> > On Mon, Aug 19, 2013 at 05:28:44PM +0800, yinyin wrote:
> > > Hi,all:
> > > 	in func virtqueue_get_avail_bytes， when found a indirect desc, we need loop over it.
> > >             /* loop over the indirect descriptor table */
> > >             indirect = 1;
> > >             max = vring_desc_len(desc_pa, i) / sizeof(VRingDesc);
> > >             num_bufs = i = 0;
> > >             desc_pa = vring_desc_addr(desc_pa, i);
> > > 	But, It init i to 0, then use i to update desc_pa. so we will always get  :
> > > 	desc_pa = vring_desc_addr(desc_pa, 0);
> > > 	is it right？or should we update desc_pa first, then init i to 0?
> > 
> > Is there a way to trigger a crash or erorr from a normal running guest?
> > 
> > Affected devices: serial, rng, and net - they call
> > virtqueue_get_avail_bytes() directly or indirectly.
> > 
> > > diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> > > index 09f62c6..554ae6f 100644
> > > --- a/hw/virtio/virtio.c
> > > +++ b/hw/virtio/virtio.c
> > > @@ -377,8 +377,8 @@ void virtqueue_get_avail_bytes(VirtQueue *vq, unsigned int *in_bytes,
> > >              /* loop over the indirect descriptor table */
> > >              indirect = 1;
> > >              max = vring_desc_len(desc_pa, i) / sizeof(VRingDesc);
> > > -            num_bufs = i = 0;
> > >              desc_pa = vring_desc_addr(desc_pa, i);
> > > +           num_bufs = i = 0;
> > 
> > I agree, this looks wrong.  git-blame(1) doesn't reveal anything
> > interesting.  Looks like this bug has been around since 2009!
> 
> Hm, why hasn't this bitten anyone yet?
> 
> 		Amit


net uses virtqueue_get_avail_bytes for RX only, and drivers
only post single buffers there.

Same seems to be true for other devices?