Re: [Qemu-devel] [PATCH v2 3/8] virtio: Add support for guest setting of queue size

[Peter Maydell]

On 25 July 2013 06:38, Michael S. Tsirkin <address@hidden> 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 <address@hidden>
>
> 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).

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.

> 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.

thanks
-- PMM