Re: [libvirt] [PATCH 0/6] RFC: qemu: virtio-{non-}transitional support
On Tue, 2019-01-15 at 17:02 +0000, Daniel P. Berrangé wrote:
On Tue, Jan 15, 2019 at 02:30:14PM +0100, Andrea Bolognani wrote:
> Basically VirtIO 0.9 requires IO space to be available, and 1.0 did
> away with that requirement because PCI Express, unlike conventional
> PCI, allows devices *not* to have IO space.
>
> So transitional devices, which must work with both 0.9 and 1.0, can
> depend on IO space being available and as such will only work when
> plugged into conventional PCI slots, whereas non-transitional
> devices don't need IO space and can thus be plugged into either
> conventional PCI and PCI Express slots.
>
> Ultimately, then, transitional (rather than non-transitional)
> devices are the ones that must be forced into conventional PCI
> slots.
Yes, the existing devices fail when placed in a PCI-X slot with certain
guest OS. The -transitional devices are functionally identical to the
existing devices. They serve as a hint to libvirt that it should never
place them in a PCI-X slot.
Not quite: existing devices (virtio-*-pci) will change their
behavior based on the slot they're plugged into, so they will show
up as non-transitional when connected to a PCI Express slot and as
transitional otherwise.
If that wasn't the case, there wouldn't have been a way to use
VirtIO devices on x86/q35 or aarch64/virt without throwing
pcie-to-pci-bridge into the mix until now, and we would also need
to change the behavior for existing devices, neither of which is
true :)
Whether or not the guest OS supports VirtIO 1.0 and can thus drive
a non-transitional device is a separate matter, which adding support
for these new devices to libvirt and libosinfo will address.
--
Andrea Bolognani / Red Hat / Virtualization