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Re: [Qemu-devel] [qemu-devel]question on virtqueue_get_avail_bytes
From: |
Michael S. Tsirkin |
Subject: |
Re: [Qemu-devel] [qemu-devel]question on virtqueue_get_avail_bytes |
Date: |
Tue, 3 Sep 2013 14:15:55 +0300 |
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?