On 04/23/2016 12:46 PM, Cole Robinson wrote:
On 04/21/2016 02:48 PM, Laine Stump wrote:
Previously there was no way to have a Q35 domain that didn't have
these two controllers. This patch skips their creation as long as
there are some other kinds of pci controllers at index 1 and 2
(e.g. some pcie-root-port controllers).
I'm hoping that soon we won't add them at all, plugging all devices
into auto-added pcie-*-port ports instead, but in the meantime this
makes it easier to experiment with alternative bus hierarchies.
---
src/qemu/qemu_domain.c | 18 +++++++++++-------
1 file changed, 11 insertions(+), 7 deletions(-)
diff --git a/src/qemu/qemu_domain.c b/src/qemu/qemu_domain.c
index 86b7d13..0b342e2 100644
--- a/src/qemu/qemu_domain.c
+++ b/src/qemu/qemu_domain.c
@@ -1581,14 +1581,18 @@ qemuDomainDefAddDefaultDevices(virDomainDefPtr def,
VIR_DOMAIN_CONTROLLER_MODEL_PCIE_ROOT)) {
goto cleanup;
}
- if (virDomainDefMaybeAddController(
- def, VIR_DOMAIN_CONTROLLER_TYPE_PCI, 1,
- VIR_DOMAIN_CONTROLLER_MODEL_DMI_TO_PCI_BRIDGE) < 0 ||
- virDomainDefMaybeAddController(
- def, VIR_DOMAIN_CONTROLLER_TYPE_PCI, 2,
- VIR_DOMAIN_CONTROLLER_MODEL_PCI_BRIDGE) < 0) {
+ /* add a dmi-to-pci-bridge and a pci-bridge if there are no pci controllers
+ * other than the pcie-root. This is so that there will be hot-pluggable
+ * PCI slots available
+ */
+ if (virDomainControllerFind(def, VIR_DOMAIN_CONTROLLER_TYPE_PCI, 1) < 0 &&
+ !virDomainDefAddController(def, VIR_DOMAIN_CONTROLLER_TYPE_PCI, 1,
+ VIR_DOMAIN_CONTROLLER_MODEL_DMI_TO_PCI_BRIDGE))
+ goto cleanup;
+ if (virDomainControllerFind(def, VIR_DOMAIN_CONTROLLER_TYPE_PCI, 2) < 0 &&
+ !virDomainDefAddController(def, VIR_DOMAIN_CONTROLLER_TYPE_PCI, 2,
+ VIR_DOMAIN_CONTROLLER_MODEL_PCI_BRIDGE))
goto cleanup;
- }
}
if (addDefaultMemballoon && !def->memballoon) {
Sounds like another qemuxml2xml test case candidate
... and it turns out this patch doesn't go far enough to have any useful
effect. The problem is that the code that automatically assigns PCI
addresses does its best to always be sure that there is at least one
empty hot-pluggable "standard PCI" slot on the system, so even if we
don't explicitly add any pci-bridge to a Q35 domain, the
address-assignment code will automatically add one anyway; and since the
pci-bridge requires a standard PCI slot itself, we still need a
dmi-to-pci-bridge anyway (which provides 32 non-hotpluggable standard
PCI slots).
I had been aiming to re-do how libvirt sets up the PCI controllers for
Q35, eventually eliminating the current dmi-to-pci-bridge + pci-bridge
in favor of a pure PCIe setup, using pcie-root-ports and
pcie-switch-(up|down)stream-ports, and this was going to be the first
step towards that. But after discussing that idea more with Alex
Williamson on Friday, I think we *shouldn't* do it, but just leave the
PCI controller setup essentially as it is (one possible change - allow
connecting pci-bridge directly into a pcie-root port, thus eliminating
the dmi-to-pci-bridge)
What prompted the idea to change to pure PCIe controllers was that qemu
allows any PCI device to be plugged into a PCIe slot and it apparently
functions just fine, and there had been occasional complaints that
plugging everything into a pci-bridge was somehow problematic (at one
point someone suggested that virtio-net didn't work correctly on ARM if
plugged into a non-Express slot, but I think that was later disproved).
The problem with PCIe-only controllers, as Alex pointed out, is that
when a non-Express device is plugged into a PCIe slot, the guest OS will
see an apparently PCIe device that has no PCIe capabilities, and while
so far this hasn't caused any problem, there is no guarantee that it
won't - PCIe devices are supposed to have PCIe capabilities. Since the
only emulated qemu device that does this is the NEC UHCI USB3
controller, it sees that we'll need to keep setting up the pci-bridge
and plugging all the rest of the devices in there.
Still, it would be nice to allow *those who really want to* to have a
"pure PCIe" controller setup. We could do that if we did two things:
1) modify the PCI address auto-assignment code to not insist on always
having at least one hot-pluggable standard PCI slot available.
and one of the following:
2a) don't always add a dmi-to-pci-bridge, but instead only do so *if
necessary* in order to plug in a pci-bridge (which would only be added
if necessary).
or
2b) permit auto-assigning a pci-bridge to plug into a port of pcie-root,
thus eliminating the need for dmi-to-pci-bridge completely.
Does anyone have opinions about (1) or (2b)?
(NB: in the long run it might be nice if qemu would automatically
advertise PCIe capabilities for any of its emulated devices that was
plugged into a PCIe slot - this would make it possible to get rid of all
non-Express controllers without violating any of the PCI specs. However,
according to Alex this would have 0 practical effect (no better
performance / actual capabilities of the devices / use of PCI address
space), and it would also require that qemu gave us some way to
understand it was going to do this (i.e. introspection) so I don't know
how useful it would be to go to all that trouble.)
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