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

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Date:   Fri, 2 Feb 2018 16:30:49 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Arnd Bergmann <arnd@...db.de>
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 02/02/18 16:29, Arnd Bergmann wrote:
> 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).

Ah, I see, that is unfortunate. Thanks for clarifying.

Robin.