linux-kernel - Re: [PATCH 1/2] ARM: kvm: fix building with gcc-8

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Date:   Fri, 2 Feb 2018 17:29:01 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Christoffer Dall <christoffer.dall@...aro.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Russell King <linux@...linux.org.uk>,
        Andi Kleen <ak@...ux.intel.com>,
        Julien Thierry <julien.thierry@....com>,
        Nicolas Pitre <nico@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "# 3.4.x" <stable@...r.kernel.org>,
        Richard Earnshaw <rearnsha@....gnu.org>,
        Tamar Christina <tnfchris@....gnu.org>,
        kvmarm@...ts.cs.columbia.edu,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 1/2] ARM: kvm: fix building with gcc-8

On Fri, Feb 2, 2018 at 5:23 PM, Robin Murphy <robin.murphy@....com> wrote:
> On 02/02/18 15:55, Robin Murphy wrote:
>>
>> On 02/02/18 15:07, Arnd Bergmann wrote:
>>>
>>> In banked-sr.c, we use a top-level '__asm__(".arch_extension virt")'
>>> statement to allow compilation of a multi-CPU kernel for ARMv6
>>> and older ARMv7-A that don't normally support access to the banked
>>> registers.
>>>
>>> This is considered to be a programming error by the gcc developers
>>> and will no longer work in gcc-8, where we now get a build error:
>>>
>>> /tmp/cc4Qy7GR.s:34: Error: Banked registers are not available with this
>>> architecture. -- `mrs r3,SP_usr'
>>> /tmp/cc4Qy7GR.s:41: Error: Banked registers are not available with this
>>> architecture. -- `mrs r3,ELR_hyp'
>>> /tmp/cc4Qy7GR.s:55: Error: Banked registers are not available with this
>>> architecture. -- `mrs r3,SP_svc'
>>> /tmp/cc4Qy7GR.s:62: Error: Banked registers are not available with this
>>> architecture. -- `mrs r3,LR_svc'
>>> /tmp/cc4Qy7GR.s:69: Error: Banked registers are not available with this
>>> architecture. -- `mrs r3,SPSR_svc'
>>> /tmp/cc4Qy7GR.s:76: Error: Banked registers are not available with this
>>> architecture. -- `mrs r3,SP_abt'
>>>
>>> Passign the '-march-armv7ve' flag to gcc works, and is ok here, because
>>> we know the functions won't ever be called on pre-ARMv7VE machines.
>>> Unfortunately, older compiler versions (4.8 and earlier) do not
>>> understand
>>> that flag, so we still need to keep the asm around.
>>>
>>> Backporting to stable kernels (4.6+) is needed to allow those to be built
>>> with future compilers as well.
>>
>>
>> Is "-Wa,arch=armv7-a+virt" (as we appear to do for a couple of files
>> already) viable as a possibly cleaner alternative, or is GCC itself now
>> policing the contents of inline asms?
>
>
> In fact, looking at the binutils history, any version capable of assembling
> this file should understand that (modulo my typo), so hopefully it ought to
> be feasible to replace these global asms with assembler flags entirely.

No, this only works for .S files, not .c, since gcc starts the output with
an explicit .arch setting that overrides the command line. I think this
was done intentionally to prevent such a hack from working, and have
more reliable checks on the validity of the assembler instruction in
inline asm statements (which we try to circumvent here).

      Arnd