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

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Date:   Sun, 4 Feb 2018 21:57:49 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Christoffer Dall <christoffer.dall@...aro.org>
Cc:     Marc Zyngier <marc.zyngier@....com>,
        Russell King <linux@...linux.org.uk>,
        Nicolas Pitre <nico@...aro.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Richard Earnshaw <rearnsha@....gnu.org>,
        Tamar Christina <tnfchris@....gnu.org>,
        "# 3.4.x" <stable@...r.kernel.org>,
        Julien Thierry <julien.thierry@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        kvmarm@...ts.cs.columbia.edu,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] ARM: kvm: fix building with gcc-8

On Sun, Feb 4, 2018 at 7:45 PM, Christoffer Dall
<christoffer.dall@...aro.org> wrote:
> Hi Arnd,
>
> On Fri, Feb 02, 2018 at 04:07:34PM +0100, 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.
>
> Does "not understand" mean "ignores" or do we get an error?

We get an error, which is why I used the $(call cc-option) Makefile
helper to check if the compiler supports it.

>> Backporting to stable kernels (4.6+) is needed to allow those to be built
>> with future compilers as well.
>
> This builds on the toolchains I have on my machine, so:
>
> Acked-by: Christoffer Dall <christoffer.dall@...aro.org>
>
> Are you applying this via a tree with other fixes or would you like me
> to carry it in the kvmarm tree?

Please pick it up in your tree.

Thanks,

       Arnd