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@xxxxxxxxxxx> 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.
