linux-kernel - Re: [PATCH 4/5] ARM: use unified assembler in c files

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Date:   Mon, 11 Feb 2019 10:17:21 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Stefan Agner <stefan@...er.ch>
Cc:     Russell King <linux@...linux.org.uk>,
        Nathan Chancellor <natechancellor@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Nicolas Pitre <nicolas.pitre@...aro.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Will Deacon <will.deacon@....com>, julien.thierry@....com,
        Mark Rutland <mark.rutland@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 4/5] ARM: use unified assembler in c files

On Sat, Feb 9, 2019 at 9:28 AM Stefan Agner <stefan@...er.ch> wrote:
>
> Thanks for the reviews so far.
>
> During some more testing I stumbled upon another issue:
>
> On 07.02.2019 10:48, Stefan Agner wrote:
> > Use unified assembler syntax (UAL) in inline assembler. Divided
> > syntax is considered depricated. This will also allow to build
> > the kernel using LLVM's integrated assembler.
> >
> > Signed-off-by: Stefan Agner <stefan@...er.ch>
> > ---
> >  arch/arm/mm/copypage-v4mc.c | 2 +-
> >  arch/arm/mm/copypage-v4wb.c | 2 +-
> >  arch/arm/mm/copypage-v4wt.c | 2 +-
> >  3 files changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/arm/mm/copypage-v4mc.c b/arch/arm/mm/copypage-v4mc.c
> > index b03202cddddb..b19c7ad1a6de 100644
> > --- a/arch/arm/mm/copypage-v4mc.c
> > +++ b/arch/arm/mm/copypage-v4mc.c
> > @@ -56,7 +56,7 @@ static void mc_copy_user_page(void *from, void *to)
> >       ldmia   %0!, {r2, r3, ip, lr}           @ 4\n\
> >       subs    %2, %2, #1                      @ 1\n\
> >       stmia   %1!, {r2, r3, ip, lr}           @ 4\n\
> > -     ldmneia %0!, {r2, r3, ip, lr}           @ 4\n\
> > +     ldmiane %0!, {r2, r3, ip, lr}           @ 4\n\
> >       bne     1b                              @ "
> >       : "+&r" (from), "+&r" (to), "=&r" (tmp)
> >       : "2" (PAGE_SIZE / 64)
> > diff --git a/arch/arm/mm/copypage-v4wb.c b/arch/arm/mm/copypage-v4wb.c
> > index cd3e165afeed..6e3c9b69dd25 100644
> > --- a/arch/arm/mm/copypage-v4wb.c
> > +++ b/arch/arm/mm/copypage-v4wb.c
> > @@ -38,7 +38,7 @@ static void v4wb_copy_user_page(void *kto, const void *kfrom)
> >       ldmia   %1!, {r3, r4, ip, lr}           @ 4\n\
> >       subs    %2, %2, #1                      @ 1\n\
> >       stmia   %0!, {r3, r4, ip, lr}           @ 4\n\
> > -     ldmneia %1!, {r3, r4, ip, lr}           @ 4\n\
> > +     ldmiane %1!, {r3, r4, ip, lr}           @ 4\n\
> >       bne     1b                              @ 1\n\
> >       mcr     p15, 0, %1, c7, c10, 4          @ 1   drain WB"
> >       : "+&r" (kto), "+&r" (kfrom), "=&r" (tmp)
> > diff --git a/arch/arm/mm/copypage-v4wt.c b/arch/arm/mm/copypage-v4wt.c
> > index 8614572e1296..4a40fa1cbc2a 100644
> > --- a/arch/arm/mm/copypage-v4wt.c
> > +++ b/arch/arm/mm/copypage-v4wt.c
> > @@ -34,7 +34,7 @@ static void v4wt_copy_user_page(void *kto, const void *kfrom)
> >       ldmia   %1!, {r3, r4, ip, lr}           @ 4\n\
> >       subs    %2, %2, #1                      @ 1\n\
> >       stmia   %0!, {r3, r4, ip, lr}           @ 4\n\
> > -     ldmneia %1!, {r3, r4, ip, lr}           @ 4\n\
> > +     ldmiane %1!, {r3, r4, ip, lr}           @ 4\n\
> >       bne     1b                              @ 1\n\
> >       mcr     p15, 0, %2, c7, c7, 0           @ flush ID cache"
> >       : "+&r" (kto), "+&r" (kfrom), "=&r" (tmp)
>
> When compiling with gcc, this leads to:
>
> /tmp/ccrvA1wy.s: Assembler messages:
> /tmp/ccrvA1wy.s:180: Error: bad instruction `ldmiane r0!,{r3,r4,ip,lr}'
> make[1]: *** [scripts/Makefile.build:277: arch/arm/mm/copypage-v4wb.o]
> Error 1
> make: *** [Makefile:1042: arch/arm/mm] Error 2
>
> Unfortunately, the gcc option -masm-syntax-unified which allows unified
> syntax in inline assembly is broken (see also
> https://lkml.org/lkml/2019/1/7/1320).
>
> So this requires to sprinkle those inline assembly with ".syntax
> unified" strings, similar to how I've done it in uaccess.h.

Not sure if this is purely a bug in GCC; Clang treats inline asm
statements as unique, so asm directives do not carry over from one
inline asm statement to another.  This has caused issues in other
arch's inline assembly, where we need to repeat asm directives
repeatedly for each inline asm statement.
https://bugs.llvm.org/show_bug.cgi?id=19749

-- 
Thanks,
~Nick Desaulniers