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Message-ID: <2325c462-07ab-4b0f-db9e-566883d5fc15@csgroup.eu>
Date: Wed, 4 Aug 2021 07:25:10 +0200
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Benjamin Herrenschmidt <benh@...nel.crashing.org>,
Paul Mackerras <paulus@...ba.org>,
Michael Ellerman <mpe@...erman.id.au>
Cc: linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] powerpc/bug: Remove specific powerpc BUG_ON() and
WARN_ON() on PPC32
Gentle ping Michael ?
Le 25/06/2021 à 16:41, Christophe Leroy a écrit :
> 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@...roup.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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