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

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Date:   Thu, 5 May 2022 11:10:28 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Niklas Schnelle <schnelle@...ux.ibm.com>,
        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>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "open list:ALPHA PORT" <linux-alpha@...r.kernel.org>,
        "moderated list:ARM PORT" <linux-arm-kernel@...ts.infradead.org>,
        "open list:IA64 (Itanium) PLATFORM" <linux-ia64@...r.kernel.org>,
        "open list:M68K ARCHITECTURE" <linux-m68k@...ts.linux-m68k.org>,
        "open list:MIPS" <linux-mips@...r.kernel.org>,
        "open list:PARISC ARCHITECTURE" <linux-parisc@...r.kernel.org>,
        "open list:LINUX FOR POWERPC (32-BIT AND 64-BIT)" 
        <linuxppc-dev@...ts.ozlabs.org>,
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        "open list:SUPERH" <linux-sh@...r.kernel.org>,
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        <sparclinux@...r.kernel.org>
Subject: Re: [RFC v2 01/39] Kconfig: introduce HAS_IOPORT option and select
 it as necessary

On Wed, May 04, 2022 at 11:31:28PM +0200, Arnd Bergmann wrote:
> On Wed, May 4, 2022 at 11:08 PM Bjorn Helgaas <helgaas@...nel.org> wrote:
> > On Fri, Apr 29, 2022 at 03:49:59PM +0200, Niklas Schnelle wrote:
> > > We introduce a new HAS_IOPORT Kconfig option to indicate support for
> > > I/O Port access. In a future patch HAS_IOPORT=n will disable compilation
> > > of the I/O accessor functions inb()/outb() and friends on architectures
> > > which can not meaningfully support legacy I/O spaces such as s390 or
> > > where such support is optional.
> >
> > So you plan to drop inb()/outb() on architectures where I/O port space
> > is optional?  So even platforms that have I/O port space may not be
> > able to use it?
> >
> > This feels like a lot of work where the main benefit is to keep
> > Kconfig from offering drivers that aren't of interest on s390.
> >
> > Granted, there may be issues where inb()/outb() does the wrong thing
> > such as dereferencing null pointers when I/O port space isn't
> > implemented.  I think that's a defect in inb()/outb() and could be
> > fixed there.
> 
> The current implementation in asm-generic/io.h implements inb()/outb()
> using readb()/writeb() with a fixed architecture specific offset.
> 
> There are three possible things that can happen here:
> 
> a) there is a host bridge driver that maps its I/O ports to this window,
>     and everything works
> b) the address range is reserved and accessible but no host bridge
>    driver has mapped its registers there, so an access causes a
>    page fault
> c) the architecture does not define an offset, and accessing low I/O
>     ports ends up as a NULL pointer dereference
> 
> The main goal is to avoid c), which is what happens on s390, but
> can also happen elsewhere. Catching b) would be nice as well,
> but is much harder to do from generic code as you'd need an
> architecture specific inline asm statement to insert a ex_table
> fixup, or a runtime conditional on each access.

Or s390 could implement its own inb().

I'm hearing that generic powerpc kernels have to run both on machines
that have I/O port space and those that don't.  That makes me think
s390 could do something similar.

Bjorn