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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


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