Re: [Qemu-devel] [PATCH V2 03/11] virito: introduce bus specific queue limit

[Cornelia Huck]

On Fri, 27 Feb 2015 03:42:00 +0008
Jason Wang <address@hidden> wrote:

> On Thu, Feb 26, 2015 at 8:57 PM, Cornelia Huck 
> <address@hidden> wrote:
> > On Thu, 26 Feb 2015 15:04:38 +0800
> > Jason Wang <address@hidden> wrote:
> > 
> >>  This patch introduces a bus specific queue limitation. It will be
> >>  useful for increasing the limit for one of the bus without 
> >> disturbing
> >>  other buses.

> >>  diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> >>  index ffc22e8..5a806b5 100644
> >>  --- a/hw/virtio/virtio.c
> >>  +++ b/hw/virtio/virtio.c
> >>  @@ -541,6 +541,14 @@ void virtio_update_irq(VirtIODevice *vdev)
> >>       virtio_notify_vector(vdev, VIRTIO_NO_VECTOR);
> >>   }
> >>  
> >>  +int virtio_get_queue_max(VirtIODevice *vdev)
> >>  +{
> >>  +    BusState *qbus = qdev_get_parent_bus(DEVICE(vdev));
> >>  +    VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(qbus);
> >>  +
> >>  +    return k->queue_max;
> >>  +}
> >>  +
> > 
> > Are all callers of this in the slow path? So we don't introduce
> > processing overhead.
> 
> Looks not. For overhead, do you mean one introduced by 
> VIRTIO_BUS_GET_CLASS()? Not sure how much it will affact but we've 
> already used something like this in the datapath, e.g 
> virtio_notify_vector(). 

I may have misremembered how much overhead those types of operation
introduce.

But it made me think: This function is basically introducing a
per-VirtIODevice queue limit. We set it once in the VirtioBusClass
during initialization, but don't expect it to change. Why don't we just
propagate it to a new member of VirtIODevice during initialization
instead?