[PATCH] vsyscall: use __iter_div_u64_rem()

[PATCH] vsyscall: use __iter_div_u64_rem()

[Arnd Bergmann]

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);
-- 
2.20.0


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Re: [PATCH] vsyscall: use __iter_div_u64_rem()

[Nathan Chancellor]

On Wed, Jul 10, 2019 at 03:01:53PM +0200, 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);
> -- 
> 2.20.0
> 

What an interesting function. Looks good to me and I can confirm it
fixes the link error.

Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Tested-by: Nathan Chancellor <natechancellor@gmail.com>

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Re: [PATCH] vsyscall: use __iter_div_u64_rem()

[Vincenzo Frascino]

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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Re: [PATCH] vsyscall: use __iter_div_u64_rem()

[Arnd Bergmann]

On Thu, Jul 11, 2019 at 2:14 PM Vincenzo Frascino
<vincenzo.frascino@arm.com> wrote:
>
>
> 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
>

See below for the patch I am using locally to work around this.
That patch is probably wrong, so I have not submitted it yet, but it
gives you a clean build ;-)

     Arnd

8<---
Subject: [PATCH] x86: percpu: fix clang 32-bit build

clang does not like an inline assembly with a "=q" contraint for
a 64-bit output:

arch/x86/events/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);
                           ^
include/linux/percpu-defs.h:447:2: note: expanded from macro '__this_cpu_read'
        raw_cpu_read(pcp);                                              \
        ^
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)
                                        ^
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>:357:1: note: expanded from here
raw_cpu_read_1
^
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)
                                        ^
arch/x86/include/asm/percpu.h:189:15: note: expanded from macro 'percpu_from_op'
                    : "=q" (pfo_ret__)                  \
                            ^

According to the commit that introduced the "q" constraint, this was
needed to fix miscompilation, but it gives no further detail.

Using the normal "=r" constraint seems to work so far.

Fixes: 3c598766a2ba ("x86: fix percpu_{to,from}_op()")
Cc: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h
index 2278797c769d..e791fbf4018f 100644
--- a/arch/x86/include/asm/percpu.h
+++ b/arch/x86/include/asm/percpu.h
@@ -99,7 +99,7 @@ do {                                                  \
        case 1:                                         \
                asm qual (op "b %1,"__percpu_arg(0)     \
                    : "+m" (var)                        \
-                   : "qi" ((pto_T__)(val)));           \
+                   : "ri" ((pto_T__)(val)));           \
                break;                                  \
        case 2:                                         \
                asm qual (op "w %1,"__percpu_arg(0)     \
@@ -144,7 +144,7 @@ do {
                         \
                else                                                    \
                        asm qual ("addb %1, "__percpu_arg(0)            \
                            : "+m" (var)                                \
-                           : "qi" ((pao_T__)(val)));                   \
+                           : "ri" ((pao_T__)(val)));                   \
                break;                                                  \
        case 2:                                                         \
                if (pao_ID__ == 1)                                      \
@@ -186,7 +186,7 @@ do {
                         \
        switch (sizeof(var)) {                          \
        case 1:                                         \
                asm qual (op "b "__percpu_arg(1)",%0"   \
-                   : "=q" (pfo_ret__)                  \
+                   : "=r" (pfo_ret__)                  \
                    : "m" (var));                       \
                break;                                  \
        case 2:                                         \
@@ -215,7 +215,7 @@ do {
                         \
        switch (sizeof(var)) {                          \
        case 1:                                         \
                asm(op "b "__percpu_arg(P1)",%0"        \
-                   : "=q" (pfo_ret__)                  \
+                   : "=r" (pfo_ret__)                  \
                    : "p" (&(var)));                    \
                break;                                  \
        case 2:                                         \

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Re: [PATCH] vsyscall: use __iter_div_u64_rem()

[Vincenzo Frascino]

Hi Arnd,

On 11/07/2019 13:28, Arnd Bergmann wrote:
> On Thu, Jul 11, 2019 at 2:14 PM Vincenzo Frascino
> <vincenzo.frascino@arm.com> wrote:
>>
>>
>> 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
>>
> 
> See below for the patch I am using locally to work around this.
> That patch is probably wrong, so I have not submitted it yet, but it
> gives you a clean build ;-)
> 
>      Arnd
> 

Thank you, I will give it a go :-)

> 8<---
> Subject: [PATCH] x86: percpu: fix clang 32-bit build
> 
> clang does not like an inline assembly with a "=q" contraint for
> a 64-bit output:
> 
> arch/x86/events/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);
>                            ^
> include/linux/percpu-defs.h:447:2: note: expanded from macro '__this_cpu_read'
>         raw_cpu_read(pcp);                                              \
>         ^
> 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)
>                                         ^
> 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>:357:1: note: expanded from here
> raw_cpu_read_1
> ^
> 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)
>                                         ^
> arch/x86/include/asm/percpu.h:189:15: note: expanded from macro 'percpu_from_op'
>                     : "=q" (pfo_ret__)                  \
>                             ^
> 
> According to the commit that introduced the "q" constraint, this was
> needed to fix miscompilation, but it gives no further detail.
> 
> Using the normal "=r" constraint seems to work so far.
> 
> Fixes: 3c598766a2ba ("x86: fix percpu_{to,from}_op()")
> Cc: Jan Beulich <jbeulich@suse.com>
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> 
> diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h
> index 2278797c769d..e791fbf4018f 100644
> --- a/arch/x86/include/asm/percpu.h
> +++ b/arch/x86/include/asm/percpu.h
> @@ -99,7 +99,7 @@ do {                                                  \
>         case 1:                                         \
>                 asm qual (op "b %1,"__percpu_arg(0)     \
>                     : "+m" (var)                        \
> -                   : "qi" ((pto_T__)(val)));           \
> +                   : "ri" ((pto_T__)(val)));           \
>                 break;                                  \
>         case 2:                                         \
>                 asm qual (op "w %1,"__percpu_arg(0)     \
> @@ -144,7 +144,7 @@ do {
>                          \
>                 else                                                    \
>                         asm qual ("addb %1, "__percpu_arg(0)            \
>                             : "+m" (var)                                \
> -                           : "qi" ((pao_T__)(val)));                   \
> +                           : "ri" ((pao_T__)(val)));                   \
>                 break;                                                  \
>         case 2:                                                         \
>                 if (pao_ID__ == 1)                                      \
> @@ -186,7 +186,7 @@ do {
>                          \
>         switch (sizeof(var)) {                          \
>         case 1:                                         \
>                 asm qual (op "b "__percpu_arg(1)",%0"   \
> -                   : "=q" (pfo_ret__)                  \
> +                   : "=r" (pfo_ret__)                  \
>                     : "m" (var));                       \
>                 break;                                  \
>         case 2:                                         \
> @@ -215,7 +215,7 @@ do {
>                          \
>         switch (sizeof(var)) {                          \
>         case 1:                                         \
>                 asm(op "b "__percpu_arg(P1)",%0"        \
> -                   : "=q" (pfo_ret__)                  \
> +                   : "=r" (pfo_ret__)                  \
>                     : "p" (&(var)));                    \
>                 break;                                  \
>         case 2:                                         \
> 

-- 
Regards,
Vincenzo

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Re: [PATCH] vsyscall: use __iter_div_u64_rem()

[Nick Desaulniers]

On Thu, Jul 11, 2019 at 5:28 AM Arnd Bergmann <arnd@arndb.de> wrote:
> clang does not like an inline assembly with a "=q" contraint for
> a 64-bit output:

Seems like starting in GCC 7, GCC may not like it either:
https://godbolt.org/z/UyBUfh
it simply warns then proceeds with code gen.  Another difference may
come from when GCC vs Clang perform dead code elimination (DCE) vs
semantic analysis.

-- 
Thanks,
~Nick Desaulniers

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Re: [PATCH] vsyscall: use __iter_div_u64_rem()

[Arnd Bergmann]

On Thu, Jul 11, 2019 at 7:14 PM 'Nick Desaulniers' via Clang Built
Linux <clang-built-linux@googlegroups.com> wrote:
>
> On Thu, Jul 11, 2019 at 5:28 AM Arnd Bergmann <arnd@arndb.de> wrote:
> > clang does not like an inline assembly with a "=q" contraint for
> > a 64-bit output:
>
> Seems like starting in GCC 7, GCC may not like it either:
> https://godbolt.org/z/UyBUfh
> it simply warns then proceeds with code gen.  Another difference may
> come from when GCC vs Clang perform dead code elimination (DCE) vs
> semantic analysis.

Right, I also had the idea to work around it with a set of
__builtin_choos_expr()
instead of the switch()/case but did not complete that patch as the percpu
code is rather complex and this would touch lots of code.

       Arnd

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