[libvirt] [PATCH v3 11/18] qemu: assign virtio devices to PCIe slot when appropriate

[Andrea Bolognani]

On Thu, 2016-10-06 at 10:34 -0400, Laine Stump wrote:
> > > +    <video>
> > > +      <model type='virtio' heads='1' primary='yes'/>
> > > +      <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x0'/>
> > 
> > I was initially baffled by this, because I expected it to
> > be assigned to one of the available pcie-root-ports just
> > like all the other virtio devices.
> > 
> > However, according to qemuDomainValidateDevicePCISlotsQ35()
> > this is intentional, so I guess we're good :)
> 
> Actually, you bring up an interesting point in light of the "Should PCIe 
> devices ever be placed directly on pcie-root?" debate on qemu-devel (I 
> think it was in the thread about the PCI topology document that Marcel 
> is writing). We currently always put the primary video device at 00:1 
> just because "we always have", and it has the nice side effect of 
> eliminating the need for legacy-PCI controllers. But in this one case 
> the device is PCIe - to follow Marcel's recommendation of putting only 
> legacy devices on pcie-root, we should be putting the virtio video 
> device on a root-port.
> 
> As I recall, Marcel and Alex were the most vocal on this subject, so I'm 
> Cc'ing them, with this bit of context - this patch auto-assigns the 
> addresses for virtio devices to be on Express ports rather than legacy 
> slots when appropriate, but there is a bit of Q35-specific code that 
> overrides any of that and always places the primary video device at 
> 00:01.0 - should we still do that for the primary video if it's virtio? 
> Or should we put it behind a root-port?

I don't think we ever got a reply... Bump? :)

-- 
Andrea Bolognani / Red Hat / Virtualization