Re: [libvirt] [PATCH 0/6] Use more PCIe, less legacy PCI slots
On Mon, 2016-08-08 at 04:56 -0400, Laine Stump wrote:
These patches use three methods to get more of the PCI devices onto
PCIe
slots on Q35 and aarch64/virt machinetypes:
1) When virtio devices can present themselves as PCIe if they're
plugged into a PCIe controller, do that. (This capability is
detected by looking for presence of the "disable-modern" option on
the virtio-net device. If you have a better idea for how to detect
it, please let me know.)
How does this play along with Ján's series[1] implementing a
way to control the protocol revision for virtio devices?
IIUC, you wouldn't set disable-modern=off or disable-legacy=on
explicitly, but rely instead on the fact that a mixed-mode
virtio device will present itself as PCIe if plugged into a
PCIe slot. Is that right?
I think we should alter the connect flags for virtio devices
based on the protocol revision, as decided by the user with
either Ján's or a comparable approach, rather than using PCIe
automatically if available.
At the very least, the current approaches don't mix - if both
your series and Ján's were to be merged now, a device that has
been configured to use virtio 0.9 would end up assigned to a
PCIe slot.
2) Any devices that aren't hotpluggable anyway will no longer
request
a hotplug-capable slot. This, along with a change to auto-assign
legacy PCI devices to pcie-root (as long as they don't require
hotplug) means the devices will now be assigned to pcie-root.
3) Also using the fact that devices that won't be hotplugged can be
assigned to pcie-root, the devices that *do* support hotplug have a
new optional subelement "<hotplug require='no'/>" (it defaults
to
"yes" for historical reasons). If hotplug require is set to 'no',
even a PCI device can be auto-assigned to pcie-root.
I haven't yet removed the dmi-to-pci-bridge that is added by default,
and we still will only auto-add pci-bridge (so if you're adding a
virtio device without <hostplug require='no'/> then you will also need
to add a <controller type='pci' model='pcie-root-port'/> to plug
it
into).
I don't get any dmi-to-pci-bridge on aarch64 virt guests,
which is good because we don't want it at the moment :)
But it doesn't get auto-added either, not even when it would
be required because I'm trying to add a legacy PCI device. So
yeah, more work needed there I guess.
[1] https://www.redhat.com/archives/libvir-list/2016-August/msg00412.html
--
Andrea Bolognani / Red Hat / Virtualization