linux-kernel - Re: [PATCH] PCI: Avoid handing out address 0 to devices

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Date:   Thu, 14 Apr 2022 15:12:39 -0700 (PDT)
From:   Palmer Dabbelt <palmer@...belt.com>
To:     macro@...am.me.uk
CC:     helgaas@...nel.org, bhelgaas@...gle.com, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject:     Re: [PATCH] PCI: Avoid handing out address 0 to devices

On Thu, 14 Apr 2022 13:22:42 PDT (-0700), macro@...am.me.uk wrote:
> 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?

I'd argue that asm-generic/io.h uses the definitions from RISC-V, but 
the result is the same ;).

The general idea is that the IO itself is pretty generic for a handful 
of ports, they just need to be decorated with some fences (or whatever 
the ISA calls them) before/after the load/store.  Those are the 
__io_p{b,a}{r,w}() macros, which are in riscv's io.h.  IIRC they stand 
for something like Port{Before,After}{Read,Write}.