linux-kernel - Re: [PATCH v3] arm64: sysreg: make mrs_s and msr

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Date:   Mon, 22 Apr 2019 10:44:10 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Arnd Bergmann <arnd@...db.de>, Mark Rutland <mark.rutland@....com>
Cc:     Kees Cook <keescook@...omium.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Alex Matveev <alxmtvv@...il.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Yury Norov <ynorov@...iumnetworks.com>,
        Matthias Kaehlcke <mka@...omium.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] arm64: sysreg: make mrs_s and msr_s macros work with
 Clang and LTO

On Tue, Apr 16, 2019 at 9:03 PM Kees Cook <keescook@...omium.org> wrote:
>
> On Tue, Apr 16, 2019 at 12:08 PM Mark Rutland <mark.rutland@....com> wrote:
> >
> > On Mon, Apr 15, 2019 at 10:22:27AM -0700, Nick Desaulniers wrote:
> > > On Mon, Apr 15, 2019 at 10:06 AM Mark Rutland <mark.rutland@....com> wrote:
> > > > It would be nice if we could simply rely on a more recent binutils these
> > > > days, which supports the generic S<op0>_<op1>_<cn>_<Cm>_<op2> sysreg
> > > > definition. That would mean we could get rid of the whole msr_s/mrs_s
> > > > hack by turning that into a CPP macro which built that name.

Mark, can you give me a test case for this? I'd like to check if
clang's integrated assembler supports this or not, so we can file an
issue and fix it if not.

> > > >
> > > > It looks like binutils has been able to do that since September 2014...
> > > >
> > > > Are folk using toolchains older than that to compile kernels?
> > >
> > > Do you have a link to a commit?  If we can pinpoint the binutils
> > > version, that might help.
> >
> > IIUC any version of binutils with commit:
> >
> >   df7b4545b2b49572 ("[PATCH/AArch64] Generic support for all system registers using mrs and msr")
> >
> > ... should be sufficent.
>
> This appears to be binutils 2.25:
>
> $ git describe --match 'binutils-2_*' --contains df7b4545b2b49572
> binutils-2_25~418
>
> Documentation/process/changes.rst lists 2.20 as current minimum for binutils.
>
> Ubuntu's old LTS (Trusty, Apr 2014) has 2.24, and that release is
> about to exit support this month.
>
> Debian's old-stable (Apr 2015) has 2.25. It's about to exit support
> when Debian Buster releases.
>
> RHEL6 (2010) has 2.20. RHEL7 (2014) has 2.25.
>
> It seems not unreasonable to require 2.25 at least for arm64 builds?

+ Arnd who has a really good feel for which distros ship what.
-- 
Thanks,
~Nick Desaulniers