Re: [Qemu-devel] [PATCH v6 12/12] virtio-blk: add x-data-plane=on|off performance feature

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Dec 18, 2012 at 03:57:17PM +0100, Stefan Hajnoczi wrote:
> > > @@ -407,6 +409,14 @@ static void virtio_blk_handle_output(VirtIODevice *vdev, VirtQueue *vq)
> > >          .num_writes = 0,
> > >      };
> > >  
> > > +    /* Some guests kick before setting VIRTIO_CONFIG_S_DRIVER_OK so start
> > > +     * dataplane here instead of waiting for .set_status().
> > > +     */
> > 
> > By the way which guests are these?
> 
> I ran a Windows 8 guest today with build 48 virtio-win drivers.  It
> notifies before the device gets its .set_status() callback invoked.
> But I could swear I've seen Linux guests do this too.


That's very broken. But looking at linux drivers it also
seems linux guests do this even today.
We have:

       err = drv->probe(dev);
        if (err)
                add_status(dev, VIRTIO_CONFIG_S_FAILED);
        else {
                add_status(dev, VIRTIO_CONFIG_S_DRIVER_OK);
                if (drv->scan)
                        drv->scan(dev);
        }

this means that unless drivers implement scan() they
will make device active before DRIVER_OK is written
as the result linux can access it and kick.
And almost no drivers implement scan.
Nasty.

Rusty, what do you think? Worth fixing?

It does mean that for now we are stuck
with a work around, but I think we need it
in virtio core in qemu, it's not dataplane
specific.

-- 
MST