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SubjectRe: [PATCH] PCI: Avoid handing out address 0 to devices
On Thu, 14 Apr 2022, Bjorn Helgaas wrote:

> > > > Address 0 is treated specially however in many places, for example in
> > > > `pci_iomap_range' and `pci_iomap_wc_range' we require that the start
> > > > address is non-zero, and even if we let such an address through, then
> > > > individual device drivers could reject a request to handle a device at
> > > > such an address, such as in `uart_configure_port'. Consequently given
> > > > devices configured as shown above only one is actually usable:
> > >
> > > pci_iomap_range() tests the resource start, i.e., the CPU address. I
> > > guess the implication is that on RISC-V, the CPU-side port address is
> > > the same as the PCI bus port address?
> >
> > Umm, for all systems I came across except x86, which have native port I/O
> > access machine instructions, a port I/O resource records PCI bus addresses
> > of the device rather than its CPU addresses, which encode the location of
> > an MMIO window the PCI port I/O space is accessed through.
>
> My point is only that it is not necessary for the PCI bus address and
> the struct resource address, i.e., the argument to inb(), to be the
> same.

Sure, but I have yet to see a system where it is the case.

Also in principle peer PCI buses could have their own port I/O address
spaces each mapped via distinct MMIO windows in the CPU address space, but
I haven't heard of such a system. That of course doesn't mean there's no
such system in existence.

> I tried to find the RISC-V definition of inb(), but it's obfuscated
> too much to be easily discoverable.

AFAICT the RISC-V port uses definitions from include/asm-generic/io.h.
Palmer, did I get this right?

Maciej

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