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

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Message-ID: <1424b9289049d0ad8b5c37a4e23ef70f0ef0f83d.camel@linux.ibm.com>
Date:   Fri, 06 May 2022 15:15:16 +0200
From:   Niklas Schnelle <schnelle@...ux.ibm.com>
To:     Arnd Bergmann <arnd@...nel.org>,
        "Maciej W. Rozycki" <macro@...am.me.uk>
Cc:     Bjorn Helgaas <helgaas@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-pci <linux-pci@...r.kernel.org>,
        Richard Henderson <rth@...ddle.net>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Michal Simek <monstr@...str.eu>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
        Helge Deller <deller@....de>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>,
        "David S. Miller" <davem@...emloft.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
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Subject: Re: [RFC v2 01/39] Kconfig: introduce HAS_IOPORT option and select
 it as necessary

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@...am.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.