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Date: Thu, 25 Jul 2013 12:03:17 +0300
From: "Michael S. Tsirkin" <mst@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v2 3/8] virtio: Add support for guest
 setting of queue size
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To: Peter Maydell <peter.maydell@linaro.org>
Cc: Anthony Liguori <aliguori@us.ibm.com>, patches@linaro.org, Alexander Graf <agraf@suse.de>, qemu-devel@nongnu.org, kvmarm@lists.cs.columbia.edu, David Gibson <david@gibson.dropbear.id.au>

On Thu, Jul 25, 2013 at 09:50:21AM +0100, Peter Maydell wrote:
> On 25 July 2013 06:38, Michael S. Tsirkin <mst@redhat.com> wrote:
> > On Fri, Jul 12, 2013 at 09:36:57PM +0100, Peter Maydell wrote:
> >> The MMIO virtio transport spec allows the guest to tell the host how
> >> large the queue size is. Add virtio_queue_set_num() function which
> >> implements this in the QEMU common virtio support code.
> >>
> >> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> >
> > Probably needs to go back to default value on reset?
> 
> Tricky, since the default value is "whatever was passed to
> virtio_add_queue()" and we don't save that anywhere.
>
> For virtio-mmio it is a guest bug to fail to write to the
> QueueNum register, so the current behaviour is not out of
> specification (and not harmful either AFAICT).

Best not to leak info across reboots.
Also if guest sets num = 0 it will cause all kind of
harm, no?

> I guess we could add a vring.defaultnum, which would be
> set by virtio_add_queue/virtio_del_queue, and have reset
> copy defaultnum into num. No migration needed for defaultnum
> because it's always the same for a particular qemu config.

Sounds good.

> > Need to migrate?
> 
> It's already migrated (though I'm not entirely sure why).
> 
> > Is the default value a max legal value? If yes probably a good
> > idea to enforce this.
> 
> virtio_add_queue() already enforces this -- it will abort()
> if you try to set up a queue with a default size greater
> than VIRTQUEUE_MAX_SIZE.

No I mean does default value have some meaning?
Does host supply a hint of a max value
besides VIRTQUEUE_MAX_SIZE?

> 
> thanks
> -- PMM