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

[Jason Wang <jasowang@redhat.com>]

On Fri, Feb 27, 2015 at 5:34 PM, Cornelia Huck 
<cornelia.huck@de.ibm.com> wrote:
> On Fri, 27 Feb 2015 03:42:00 +0008
> Jason Wang <jasowang@redhat.com> wrote:
> 
>>  On Thu, Feb 26, 2015 at 8:57 PM, Cornelia Huck 
>>  <cornelia.huck@de.ibm.com> wrote:
>>  > On Thu, 26 Feb 2015 15:04:38 +0800
>>  > Jason Wang <jasowang@redhat.com> 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?

The limit may be changed. Consider that patch 9 increases the pci limit 
from 64 to 513. But we want to keep the migration compatibility for 
legacy machine types e.g machines earlier than pc-q35-2.3 or 
pc-i440fx-2.3, so the limit was changed back to 64 in pc_compat_2_2(). 
More work will be done if we want to propagate it to VirtIODevice, and 
it looks unnecessary.