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

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Message-ID: <20180205083316.GR21802@cbox>
Date:   Mon, 5 Feb 2018 09:33:16 +0100
From:   Christoffer Dall <christoffer.dall@...aro.org>
To:     Arnd Bergmann <arnd@...db.de>
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 04, 2018 at 09:57:49PM +0100, Arnd Bergmann wrote:
> 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.
> 

Right.

> >> 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.
> 
Will do.

Thanks,
-Christoffer