Re: [RFC v2 01/39] Kconfig: introduce HAS_IOPORT option and select it as necessary

Niklas Schnelle
2022-05-06T13:16:45Z

On Fri, 2022-05-06 at 14:53 +0200, Arnd Bergmann wrote:
> On Fri, May 6, 2022 at 2:27 PM Maciej W. Rozycki <macro@orcam.me.uk> wrote:
> > On Fri, 6 May 2022, Arnd Bergmann wrote:
> >
> > > > If this is PCI/PCIe indeed, then an I/O access is just a different bit
> > > > pattern put on the bus/in the TLP in the address phase. So what is there
> > > > inherent to the s390 architecture that prevents that different bit pattern
> > > > from being used?
> > >
> > > The hardware design for PCI on s390 is very different from any other
> > > architecture, and more abstract. Rather than implementing MMIO register
> > > access as pointer dereference, this is a separate CPU instruction that
> > > takes a device/bar plus offset as arguments rather than a pointer, and
> > > Linux encodes this back into a fake __iomem token.
> >
> > OK, that seems to me like a reasonable and quite a clean design (on the
> > hardware side).
> >
> > So what happens if the instruction is given an I/O rather than memory BAR
> > as the relevant argument? Is the address space indicator bit (bit #0)
> > simply ignored or what?
>
> Not sure. My best guess is that it would actually work as you'd expect,
> but is deliberately left out of the architecture specification so they don't
> have to to validate the correctness. Note that only a small number of
> PCIe cards are actually supported by IBM, and I think the firmware
> only passes devices to the OS if they are whitelisted.
>
> Arnd

Yes, though in Linux we do try hard to work with whatever is plugged
in. We did benefit from this in the past working with a new NIC from a
different vendor with 0 additional changes. Also you can use vfio-pci
to pass-through arbitrary PCI devices to a QEMU emulating s390x.