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SubjectRe: [PATCH 1/2] powerpc/bug: Remove specific powerpc BUG_ON() and WARN_ON() on PPC32
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Hi Michael,

What happened to this series ? It has been flagged 'under review' in Patchwork since mid April but I
never saw it in next-test.

Thanks
Christophe

Le 12/04/2021 à 18:26, Christophe Leroy a écrit :
> powerpc BUG_ON() and WARN_ON() are based on using twnei instruction.
>
> For catching simple conditions like a variable having value 0, this
> is efficient because it does the test and the trap at the same time.
> But most conditions used with BUG_ON or WARN_ON are more complex and
> forces GCC to format the condition into a 0 or 1 value in a register.
> This will usually require 2 to 3 instructions.
>
> The most efficient solution would be to use __builtin_trap() because
> GCC is able to optimise the use of the different trap instructions
> based on the requested condition, but this is complex if not
> impossible for the following reasons:
> - __builtin_trap() is a non-recoverable instruction, so it can't be
> used for WARN_ON
> - Knowing which line of code generated the trap would require the
> analysis of DWARF information. This is not a feature we have today.
>
> As mentioned in commit 8d4fbcfbe0a4 ("Fix WARN_ON() on bitfield ops")
> the way WARN_ON() is implemented is suboptimal. That commit also
> mentions an issue with 'long long' condition. It fixed it for
> WARN_ON() but the same problem still exists today with BUG_ON() on
> PPC32. It will be fixed by using the generic implementation.
>
> By using the generic implementation, gcc will naturally generate a
> branch to the unconditional trap generated by BUG().
>
> As modern powerpc implement zero-cycle branch,
> that's even more efficient.
>
> And for the functions using WARN_ON() and its return, the test
> on return from WARN_ON() is now also used for the WARN_ON() itself.
>
> On PPC64 we don't want it because we want to be able to use CFAR
> register to track how we entered the code that trapped. The CFAR
> register would be clobbered by the branch.
>
> A simple test function:
>
> unsigned long test9w(unsigned long a, unsigned long b)
> {
> if (WARN_ON(!b))
> return 0;
> return a / b;
> }
>
> Before the patch:
>
> 0000046c <test9w>:
> 46c: 7c 89 00 34 cntlzw r9,r4
> 470: 55 29 d9 7e rlwinm r9,r9,27,5,31
> 474: 0f 09 00 00 twnei r9,0
> 478: 2c 04 00 00 cmpwi r4,0
> 47c: 41 82 00 0c beq 488 <test9w+0x1c>
> 480: 7c 63 23 96 divwu r3,r3,r4
> 484: 4e 80 00 20 blr
>
> 488: 38 60 00 00 li r3,0
> 48c: 4e 80 00 20 blr
>
> After the patch:
>
> 00000468 <test9w>:
> 468: 2c 04 00 00 cmpwi r4,0
> 46c: 41 82 00 0c beq 478 <test9w+0x10>
> 470: 7c 63 23 96 divwu r3,r3,r4
> 474: 4e 80 00 20 blr
>
> 478: 0f e0 00 00 twui r0,0
> 47c: 38 60 00 00 li r3,0
> 480: 4e 80 00 20 blr
>
> So we see before the patch we need 3 instructions on the likely path
> to handle the WARN_ON(). With the patch the trap goes on the unlikely
> path.
>
> See below the difference at the entry of system_call_exception where
> we have several BUG_ON(), allthough less impressing.
>
> With the patch:
>
> 00000000 <system_call_exception>:
> 0: 81 6a 00 84 lwz r11,132(r10)
> 4: 90 6a 00 88 stw r3,136(r10)
> 8: 71 60 00 02 andi. r0,r11,2
> c: 41 82 00 70 beq 7c <system_call_exception+0x7c>
> 10: 71 60 40 00 andi. r0,r11,16384
> 14: 41 82 00 6c beq 80 <system_call_exception+0x80>
> 18: 71 6b 80 00 andi. r11,r11,32768
> 1c: 41 82 00 68 beq 84 <system_call_exception+0x84>
> 20: 94 21 ff e0 stwu r1,-32(r1)
> 24: 93 e1 00 1c stw r31,28(r1)
> 28: 7d 8c 42 e6 mftb r12
> ...
> 7c: 0f e0 00 00 twui r0,0
> 80: 0f e0 00 00 twui r0,0
> 84: 0f e0 00 00 twui r0,0
>
> Without the patch:
>
> 00000000 <system_call_exception>:
> 0: 94 21 ff e0 stwu r1,-32(r1)
> 4: 93 e1 00 1c stw r31,28(r1)
> 8: 90 6a 00 88 stw r3,136(r10)
> c: 81 6a 00 84 lwz r11,132(r10)
> 10: 69 60 00 02 xori r0,r11,2
> 14: 54 00 ff fe rlwinm r0,r0,31,31,31
> 18: 0f 00 00 00 twnei r0,0
> 1c: 69 60 40 00 xori r0,r11,16384
> 20: 54 00 97 fe rlwinm r0,r0,18,31,31
> 24: 0f 00 00 00 twnei r0,0
> 28: 69 6b 80 00 xori r11,r11,32768
> 2c: 55 6b 8f fe rlwinm r11,r11,17,31,31
> 30: 0f 0b 00 00 twnei r11,0
> 34: 7d 8c 42 e6 mftb r12
>
> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
> ---
> arch/powerpc/include/asm/bug.h | 9 ++++++---
> 1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/bug.h b/arch/powerpc/include/asm/bug.h
> index d1635ffbb179..101dea4eec8d 100644
> --- a/arch/powerpc/include/asm/bug.h
> +++ b/arch/powerpc/include/asm/bug.h
> @@ -68,7 +68,11 @@
> BUG_ENTRY("twi 31, 0, 0", 0); \
> unreachable(); \
> } while (0)
> +#define HAVE_ARCH_BUG
> +
> +#define __WARN_FLAGS(flags) BUG_ENTRY("twi 31, 0, 0", BUGFLAG_WARNING | (flags))
>
> +#ifdef CONFIG_PPC64
> #define BUG_ON(x) do { \
> if (__builtin_constant_p(x)) { \
> if (x) \
> @@ -78,8 +82,6 @@
> } \
> } while (0)
>
> -#define __WARN_FLAGS(flags) BUG_ENTRY("twi 31, 0, 0", BUGFLAG_WARNING | (flags))
> -
> #define WARN_ON(x) ({ \
> int __ret_warn_on = !!(x); \
> if (__builtin_constant_p(__ret_warn_on)) { \
> @@ -93,9 +95,10 @@
> unlikely(__ret_warn_on); \
> })
>
> -#define HAVE_ARCH_BUG
> #define HAVE_ARCH_BUG_ON
> #define HAVE_ARCH_WARN_ON
> +#endif
> +
> #endif /* __ASSEMBLY __ */
> #else
> #ifdef __ASSEMBLY__
>

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