Re: [RFC 1/4] New virtio bus driver

[Arnd Bergmann <arnd@arndb.de>]

On Monday 09 July 2007, Avi Kivity wrote:
> > That is much harder to do, it would require a separate interrupt if you
> > want to have a device independent mechanism. Most real buses (like PCI)
> > don't have this, so I'm not sure it's good to do this at the virtio
> > layer. You already listed the media change notification, which can
> > be handled without this by either removing and adding the complete
> > virtio device, or by using a channel for out-of-band data like scsi.
> 
> At the very least, we need an interrupt for device hotplug.  That same
> interrupt can be used to notify the guest about configuration changes
> within a device ("rescan the virtbus and all who sail in her").

There is an important difference between device reconfiguration and bus
reconfiguration (device hotplug). If we want device reconfiguration, that
needs to be implemented within the virtio layer, because the device
drivers need to know about it. For hotplugging devices, we need to have
a bus specific mechanism, while the device drivers only care about
'new device found' and 'device is gone' events that are already supported
by any linux bus_type.

The problem with hotplugging devices on PCI is that there is not a good
standard for this yet. The best option we have is probably PCI express
hotplug, but I'm not sure how many legacy operating systems already support
this.

	Arnd <><