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The new interface uses pci capability structures to locate the
registers in a rather fine granular way making it more complicated
as they can be located anywhere in any pci bar at any offset.
As far as i can see, qemu (6.0.50) never uses i/o bars in
non-legacy mode, so only mmio is implemented for now.
The previous virtio drivers implemented the legacy interface only
which uses i/o ports for all register accesses. This is still
the preferred method (and also qemu default) as it is easier to
emulate and most likely faster.
However, some vps providers like vultr force the legacy interface
to disabled with qemu -device option "disable-legacy=on" resulting
on a system without a disk and ethernet.
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