linux-kernel - Re: [PATCH] [v2] coresight: etm4x: avoid build failure with unrolled loops

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Date:   Thu, 29 Apr 2021 23:49:53 +0100
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Mathieu Poirier <mathieu.poirier@...aro.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Mike Leach <mike.leach@...aro.org>,
        Leo Yan <leo.yan@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>,
        Qi Liu <liuqi115@...wei.com>,
        Tingwei Zhang <tingwei@...eaurora.org>,
        coresight@...ts.linaro.org,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] [v2] coresight: etm4x: avoid build failure with unrolled
 loops

On 29/04/2021 18:50, Arnd Bergmann wrote:
> On Thu, Apr 29, 2021 at 7:37 PM Suzuki K Poulose <suzuki.poulose@....com> wrote:
>> On 29/04/2021 15:57, Arnd Bergmann wrote:
>>> From: Arnd Bergmann <arnd@...db.de>
>>>
>>> clang-12 fails to build the etm4x driver with -fsanitize=array-bounds,
>>> where it decides to unroll certain loops in a way that result in a
>>> C variable getting put into an inline assembly
>>>
>>> <instantiation>:1:7: error: expected constant expression in '.inst' directive
>>> .inst (0xd5200000|((((2) << 19) | ((1) << 16) | (((((((((((0x160 + (i * 4))))) >> 2))) >> 7) & 0x7)) << 12) | ((((((((((0x160 + (i * 4))))) >> 2))) & 0xf)) << 8) | (((((((((((0x160 + (i * 4))))) >> 2))) >> 4) & 0x7)) << 5)))|(.L__reg_num_x8))
>>>         ^
>>> drivers/hwtracing/coresight/coresight-etm4x-core.c:702:4: note: while in macro instantiation
>>>                           etm4x_relaxed_read32(csa, TRCCNTVRn(i));
>>>                           ^
>>> drivers/hwtracing/coresight/coresight-etm4x.h:403:4: note: expanded from macro 'etm4x_relaxed_read32'
>>>                    read_etm4x_sysreg_offset((offset), false)))
>>>                    ^
>>> drivers/hwtracing/coresight/coresight-etm4x.h:383:12: note: expanded from macro 'read_etm4x_sysreg_offset'
>>>                           __val = read_etm4x_sysreg_const_offset((offset));       \
>>>                                   ^
>>> drivers/hwtracing/coresight/coresight-etm4x.h:149:2: note: expanded from macro 'read_etm4x_sysreg_const_offset'
>>>           READ_ETM4x_REG(ETM4x_OFFSET_TO_REG(offset))
>>>           ^
>>> drivers/hwtracing/coresight/coresight-etm4x.h:144:2: note: expanded from macro 'READ_ETM4x_REG'
>>>           read_sysreg_s(ETM4x_REG_NUM_TO_SYSREG((reg)))
>>>           ^
>>> arch/arm64/include/asm/sysreg.h:1108:15: note: expanded from macro 'read_sysreg_s'
>>>           asm volatile(__mrs_s("%0", r) : "=r" (__val));                  \
>>>                        ^
>>> arch/arm64/include/asm/sysreg.h:1074:2: note: expanded from macro '__mrs_s'
>>> "       mrs_s " v ", " __stringify(r) "\n"                      \
>>>    ^
>>>
>>> This only happened in a few loops in which the array bounds sanitizer
>>> added a special case for an array overflow that clang determined to be
>>> possible, but any compiler is free to unroll any of the loops in the
>>> same way that breaks the sysreg macros.
>>>
>>> Introduce helper functions that perform a sysreg access with a
>>> non-constant register number and use them in each call that passes
>>> a loop counter.
>>
>> You don't need to add this special helper. We have the exact
>> infrastructure already. So these could simply be replaced with:
>>
>> csdev_access_xxx(csa, ...)
>>
>> see :
>>
>> include/linux/coresight.h
> 
> Ah, nice!
> 
> Do you mean replacing only the ones that use a nonconstant
> offset, or all of them? I guess changing all would avoid some

Only the CLANG "nonconstant" please. Going through an indirect
function call and large "switch..case" for every single register
access is painful. Which is why I decided to add those ugly macros.

So, please leave the rest as they are.

Suzuki


> really ugly magic macros, but the indirect function call and the
> switch() adds a few cycles of overhead every time and the code
> looks like it is micro-optimized for fast register access here.
> 
>        Arnd
>