On Wed, Oct 06, 2010 at 05:24:18PM +0530, Amit Shah wrote:
> On (Tue) Oct 05 2010 [17:23:19], Michael S. Tsirkin wrote:
> > On Tue, Oct 05, 2010 at 07:15:31PM +0530, Amit Shah wrote:
> > > > > +
> > > > > + spin_lock_irqsave(&vp_dev->lock, flags);
> > > > > + list_for_each_entry(info, &vp_dev->virtqueues, node) {
> > > > > + /* Select the queue we're interested in */
> > > > > + iowrite16(info->queue_index,
> > > > > + vp_dev->ioaddr + VIRTIO_PCI_QUEUE_SEL);
> > > > > +
> > > > > + /* Update the last idx we sent data in */
> > > > > + iowrite16(virtqueue_get_avail_idx(info->vq),
> > > > > + vp_dev->ioaddr + VIRTIO_PCI_AVAIL_IDX);
> > > >
> > > > Interesting. Could we just reset the ring instead?
> > > > I think this would also solve the qemu problem you
> > > > outline, won't it?
> > >
> > > The problem here is qemu doesn't "know" we went into suspend and came
> > > out of it. When going to suspend, qemu could've received a kick
> > > notification and would've been just about to process some queue entries.
> > > Now when we resume and reset the ring, qemu could crash anyway seeing
> > > invalid index values.
> >
> > Hmm, I don't completely understand this. When there's a reset I expect
> > this to discard any previous kicks. No?
>
> I'm talking of a situation like this:
>
>
> Guest Host
>
> virtqueue_add_buf()
> virtqueue_kick()
> virtqueue_pop() (in progress)
>
> --> suspend
>
>
> Now there will be some buffers in the virtqueue but the host wouldn't
> know on the next resume. So we want to keep the ring state in the
> current state so that the host continues consuming from where it left
> off.
I still don't see it. why don't we reset on resume?
If there's a reset host must either discard or
flush out operations in progress.
> Now this wouldn't crash on resume for virtio-net, but for virtio-serial
> (which uses chardevs) and also for virtio-block, I guess there are more
> problems.
>
> For example, for virtio-serial, if the machine is re-started with a
> chardev connected, the virtqueue_num_heads() function gets called, which
> results in the 'Guest moved used index' message.
>
> Amit
Guest moved used index means a vring related bug?
When the ring is reset both host and guest should start from 0,
and index must be 0 too.
--
MST
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