Re: [PATCH V13 - RESEND 02/10] arm64/perf: Add BRBE registers and fields

[Anshuman Khandual]

On 7/28/23 22:22, James Clark wrote:
> 
> 
> On 28/07/2023 17:20, Will Deacon wrote:
>> On Tue, Jul 11, 2023 at 01:54:47PM +0530, Anshuman Khandual wrote:
>>> This adds BRBE related register definitions and various other related field
>>> macros there in. These will be used subsequently in a BRBE driver which is
>>> being added later on.
>>>
>>> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
>>> Cc: Will Deacon <will@xxxxxxxxxx>
>>> Cc: Marc Zyngier <maz@xxxxxxxxxx>
>>> Cc: Mark Rutland <mark.rutland@xxxxxxx>
>>> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
>>> Cc: linux-kernel@xxxxxxxxxxxxxxx
>>> Tested-by: James Clark <james.clark@xxxxxxx>
>>> Reviewed-by: Mark Brown <broonie@xxxxxxxxxx>
>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
>>> ---
>>>  arch/arm64/include/asm/sysreg.h | 103 +++++++++++++++++++++
>>>  arch/arm64/tools/sysreg         | 158 ++++++++++++++++++++++++++++++++
>>>  2 files changed, 261 insertions(+)
>>>
>>> diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
>>> index b481935e9314..f95e30c13c8b 100644
>>> --- a/arch/arm64/include/asm/sysreg.h
>>> +++ b/arch/arm64/include/asm/sysreg.h
>>> @@ -163,6 +163,109 @@
>>>  #define SYS_DBGDTRTX_EL0		sys_reg(2, 3, 0, 5, 0)
>>>  #define SYS_DBGVCR32_EL2		sys_reg(2, 4, 0, 7, 0)
>>>  
>>> +#define __SYS_BRBINFO(n)		sys_reg(2, 1, 8, ((n) & 0xf), ((((n) & 0x10)) >> 2 + 0))
>>> +#define __SYS_BRBSRC(n)			sys_reg(2, 1, 8, ((n) & 0xf), ((((n) & 0x10)) >> 2 + 1))
>>> +#define __SYS_BRBTGT(n)			sys_reg(2, 1, 8, ((n) & 0xf), ((((n) & 0x10)) >> 2 + 2))
>>
>> It's that time on a Friday but... aren't these macros busted? I think you
>> need brackets before adding the offset, otherwise wouldn't, for example,
>> target registers 0-15 all access info register 0 and __SYS_BRBTGT(16) would
>> then start accessing source register 0?
>>
>> I'm surprised that the compiler doesn't warn about this, but even more
>> surprised that you managed to test this.
>>
>> Please tell me I'm wrong!
>>
>> Will
> 
> No I think you are right, it is wrong. Luckily there is already an
> extraneous bracket so you you can fix it by moving one a place down:
> 
>   sys_reg(2, 1, 8, ((n) & 0xf), ((((n) & 0x10) >> 2) + 2))
> 
> It's interesting because the test [1] is doing quite a bit and looking
> at the branch info, and that src and targets match up to function names.
> I also manually looked at the branch buffers and didn't see anything
> obviously wrong like things that looked like branch infos in the source
> or target fields. Will have to take another look to see if it would be
> possible for the test to catch this.
> 
> James
> 
> [1]:
> https://gitlab.arm.com/linux-arm/linux-jc/-/commit/3a7ddce70c2daadb63fcc511de0a89055ca48b32

((((n) & 0x10)) >> 2 + 2) ---> ((((n) & 0x10) >> 2) + 2)

The additional brackets are useful in explicitly telling the compiler but
what it the compiler is just doing the right thing implicitly i.e computing
the shifting operation before doing the offset addition. During testing, all
those captured branch records looked alright. But that is no excuse, for not
doing the right thing to begin with i.e adding explicit brackets. I will fix
these in next version.