Re: [RFC PATCH 0/2] PCI bus/CPU address translation for MMIO

[Jinyu Tang <jinyu.tang@linux.dev>]

On Wed, Jul 15, 2026 at 04:47:47PM +0100, Will Deacon wrote:
> On Wed, Jul 01, 2026 at 10:26:52PM +0800, Jinyu Tang wrote:
> > This series adds PCI bus/CPU address translation hooks with an identity
> > default, then wires up RISC-V to exercise a non-identity PCI MMIO window
> > through a new --pci-mmio-cpu-base option.
>
> Why is that something a user would want to do?

I added it as a simple way to exercise a non-identity PCI host bridge
window without adding a new machine type.

The main case is for VFIO MMIO. PCI BAR values are bus addresses,
while KVM installs VFIO BAR mappings/traps at guest CPU physical
addresses. Linux appears to make the same distinction when probing BARs:
pcibios_bus_to_resource() converts the BAR bus address to the CPU
resource address before it is used. Some real systems also appear to have
such non-identity PCI host bridge windows, for example the PCIe host
described in arch/arm64/boot/dts/hisilicon/hi3660.dtsi.

I added the option in this RFC to help validate kvmtool with our
in-development platform model, but I can remove it in v2. I think
kvmtool can add the internal PCI bus/CPU translation for non-identity
PCI windows.

Thanks,
Jinyu