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

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Date:   Thu, 14 Apr 2022 21:22:42 +0100 (BST)
From:   "Maciej W. Rozycki" <macro@...am.me.uk>
To:     Bjorn Helgaas <helgaas@...nel.org>,
        Palmer Dabbelt <palmer@...belt.com>
cc:     Bjorn Helgaas <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, 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