Re: Configuration vs. compat hints [was Re: [Qemu-devel] [PATCHv3 03/13] qemu: add routines to manage PCI capabilities]

[Anthony Liguori]

Avi Kivity wrote:
> On 06/15/2009 03:52 PM, Anthony Liguori wrote:
> > Avi Kivity wrote:
> > > On 06/15/2009 03:41 PM, Michael S. Tsirkin wrote:
> > > > We should just tell the user which slots are open.
> > > >   This might be tricky if the config is passed in with the command 
> > > > line
> > > > flags.
> > >
> > > qemu -show-available-pci-slots
> >
> > Why does the user care?
> >
> > Let QEMU allocate the PCI slot, then query it to see what slot it 
> > assigned and remember that.
>
> It's a roundabout way of doing things.

Having libvirt do PCI slot allocation scares me.  It assumes we can 
return a whitelist of available slots, and then let libvirt just 
randomly assign things.  There's knowledge though in slot assignment 
that's board-specific.  For instance, depending on how many LNK lines 
you have, you may want to put things in slots in such a way to optimize 
interrupt balancing or something like that.

Some platforms have quirks about expecting a particular slot to have a 
particular device.  It's still an optimal device but it has to be in 
that slot.  You can't really express that via an available slot list.

> Non-virtio, as well.  But we can't make that the default, so the user 
> will have to specify this anyway.
>
> Given that you can't hotunplug individual functions, the user will 
> have to specify exactly how functions are aggregated into devices.  My 
> recommendation would be for a GUI to allow the user to select a 'quad 
> port virtio NIC' or 'dual port virtio scsi controller' rather than 
> trying to do anything automatic.

Yeah, I haven't thought much about that.

Regards,

Anthony Liguori