On 11/15/2016 07:21 AM, Andrea Bolognani wrote:
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? :)
I looked at "lspci" of a guest that has virtio-gpu directly connected to pci-root, and found that it does show up as a legacy device in that case, which I guess is what we want, right? Let's ask the author of virtio-gpu: Gerd, what is your opinion - when using virtio-gpu as the primary video device, is it better to have it directly plugged into 00:01.0 (i.e. a legacy integrated device)? Or is it better to plug it into a pcie-root-port?