linux-kernel - Re: [PATCH] vsyscall: use __iter_div_u64

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Message-ID: <33511b0e-6d7b-c156-c415-7a609b049567@arm.com>
Date:   Thu, 11 Jul 2019 13:14:39 +0100
From:   Vincenzo Frascino <vincenzo.frascino@....com>
To:     Arnd Bergmann <arnd@...db.de>, Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-mips@...r.kernel.org, linux-kselftest@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Russell King <linux@...linux.org.uk>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Paul Burton <paul.burton@...s.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Mark Salyzyn <salyzyn@...roid.com>,
        Peter Collingbourne <pcc@...gle.com>,
        Shuah Khan <shuah@...nel.org>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Huw Davies <huw@...eweavers.com>, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH] vsyscall: use __iter_div_u64_rem()

Hi Arnd,

On 10/07/2019 14:01, Arnd Bergmann wrote:
> On 32-bit x86 when building with clang-9, the loop gets turned back into
> an inefficient division that causes a link error:
> 
> kernel/time/vsyscall.o: In function `update_vsyscall':
> vsyscall.c:(.text+0xe3): undefined reference to `__udivdi3'
> 
> Use the provided __iter_div_u64_rem() function that is meant to address
> the same case in other files.
> 
> Fixes: 44f57d788e7d ("timekeeping: Provide a generic update_vsyscall() implementation")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  kernel/time/vsyscall.c | 6 +-----
>  1 file changed, 1 insertion(+), 5 deletions(-)
> 
> diff --git a/kernel/time/vsyscall.c b/kernel/time/vsyscall.c
> index a80893180826..8cf3596a4ce6 100644
> --- a/kernel/time/vsyscall.c
> +++ b/kernel/time/vsyscall.c
> @@ -104,11 +104,7 @@ void update_vsyscall(struct timekeeper *tk)
>  	vdso_ts->sec	= tk->xtime_sec + tk->wall_to_monotonic.tv_sec;
>  	nsec		= tk->tkr_mono.xtime_nsec >> tk->tkr_mono.shift;
>  	nsec		= nsec + tk->wall_to_monotonic.tv_nsec;
> -	while (nsec >= NSEC_PER_SEC) {
> -		nsec = nsec - NSEC_PER_SEC;
> -		vdso_ts->sec++;
> -	}
> -	vdso_ts->nsec	= nsec;
> +	vdso_ts->sec	+= __iter_div_u64_rem(nsec, NSEC_PER_SEC, &vdso_ts->nsec);
>  
>  	if (__arch_use_vsyscall(vdata))
>  		update_vdso_data(vdata, tk);
> 

I am trying to test this patch using clang-9 tip:

# clang -v
clang version 9.0.0 (git@...hub.com:llvm-mirror/clang.git
6ed0749151866894a67a3e7eefdc1f3a547daa0e) (git@...hub.com:llvm-mirror/llvm.git
a10a70238ace1093cad3adeb94814b422bd1b5c1)

but I get a lot of errors similar to the one below:

In file included from ~/linux/arch/x86/events/amd/core.c:11:
~/linux/arch/x86/events/amd/../perf_event.h:824:21: error: invalid output size
for constraint '=q'
        u64 disable_mask = __this_cpu_read(cpu_hw_events.perf_ctr_virt_mask);
                           ^
~/linux/include/linux/percpu-defs.h:447:2: note: expanded from macro
'__this_cpu_read'
        raw_cpu_read(pcp);                                              \
        ^
~/linux/include/linux/percpu-defs.h:421:28: note: expanded from macro 'raw_cpu_read'
#define raw_cpu_read(pcp)               __pcpu_size_call_return(raw_cpu_read_, pcp)
                                        ^
~/linux/include/linux/percpu-defs.h:322:23: note: expanded from macro
'__pcpu_size_call_return'
        case 1: pscr_ret__ = stem##1(variable); break;                  \
                             ^
<scratch space>:110:1: note: expanded from here
raw_cpu_read_1
^
~/linux/arch/x86/include/asm/percpu.h:394:30: note: expanded from macro
'raw_cpu_read_1'
#define raw_cpu_read_1(pcp)             percpu_from_op(, "mov", pcp)
                                        ^
~/linux/arch/x86/include/asm/percpu.h:189:15: note: expanded from macro
'percpu_from_op'
                    : "=q" (pfo_ret__)                  \

Could you please tell me which version of the compiler did you use?

My building command is:

# make mrproper && make CC=clang HOSTCC=clang i386_defconfig && make ARCH=i386
CC=clang HOSTCC=clang -j56

-- 
Regards,
Vincenzo