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

On Tue, 2016-09-20 at 15:14 -0400, Laine Stump wrote: > libvirt previously assigned nearly all devices to a "hotpluggable" > legacy PCI slot even on machines with a PCIe root bus (and even though > most such machines don't even support hotplug on legacy PCI slots!) > Forcing all devices onto legacy PCI slots means that the domain will > need a dmi-to-pci-bridge (to convert from PCIe to legacy PCI) and a > pci-bridge (to provide hotpluggable legacy PCI slots which, again, > usually aren't hotpluggable anyway). > > To help reduce the need for these legacy controllers, this patch tries > to assign virtio-1.0-capable devices to PCIe slots whenever possible, > by setting appropriate connectFlags in > virDomainDeviceConnectFlagsInternal(). Happily, when that function was > written (just a few commits ago) it was created with a "virtioFlags" > argument, set by both of its callers, which is the proper connectFlags > to set for any virtio-*-pci device - depending on the arch/machinetype > of the domain, and whether or not the qemu binary supports virtio-1.0, > that flag will have either been set to PCI or PCIE. This patch merely > enables the functionality by setting the flags for the device to > whatever is in virtioFlags if the device is a virtio-*-pci device. > > NB: since the slot must be hotpluggable, and pcie-root (the PCIe root > complex) does *not* support hotplug, this means that suitable > controllers must also be in the config (i.e. either pcie-root-port, or > pcie-downstream-port). For now, libvirt doesn't add those > automatically, so if you put virtio devices in a config for a qemu > that has PCIe-capable virtio devices, you'll need to add extra > pcie-root-ports yourself. That requirement will be eliminated in a > future patch, but for now, it's simple to do this: > > <controller type='pci' model='pcie-root-port'/> > <controller type='pci' model='pcie-root-port'/> > <controller type='pci' model='pcie-root-port'/> > ... > > Partially Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1330024 [...] > + <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 :)

One improvement I can recommend to this test case is to add one more pcie-root-port, so that it becomes quite clear that virtio-vga has been assigned to pcie-root on purpose and *not* due to a lack of pcie-root-ports.