Modern virtio PCI is allowed to use both memory and I/O BARs for the
config space, but legacy devices must use I/O for BAR0, as specified by
Virtio v1.0 cs04:

4.1.5.1.1.1 Legacy Interface: A Note on Device Layout Detection
"Transitional devices MUST expose the Legacy Interface in I/O space in
BAR0."

What virtio calls "I/O space" is most certainly port I/O, as hinted by the
discussion in 4.1.4 Virtio Structure PCI Capabilities, where it
distinguishes "memory BARs" from "I/O BARs". This is also the conclusion
made by SeaBIOS [1], which only looks for port I/O in BAR0 when driving a
transitional device.

I think MMIO was made the default by a463650c ("kvm tools: pci: add
MMIO interface to virtio-pci devices") to support ARM targets, but we
support PIO as well as MMIO nowadays. So let's make the legacy virtio
implementation comply with the specification and use port I/O for BAR0.

[1] https://patchwork.kernel.org/patch/10038927/



Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>