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SubjectRe: [PATCH] vsyscall: use __iter_div_u64_rem()
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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@arndb.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@github.com:llvm-mirror/clang.git
6ed0749151866894a67a3e7eefdc1f3a547daa0e) (git@github.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

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