linux-kernel - Re: [PATCH v7 4/5] x86: perf: Refactor misc flag assignments

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Message-ID: <gsntbjypft37.fsf@coltonlewis-kvm.c.googlers.com>
Date: Fri, 08 Nov 2024 19:01:16 +0000
From: Colton Lewis <coltonlewis@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: kvm@...r.kernel.org, oliver.upton@...ux.dev, seanjc@...gle.com, 
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Subject: Re: [PATCH v7 4/5] x86: perf: Refactor misc flag assignments

Peter Zijlstra <peterz@...radead.org> writes:

> On Thu, Nov 07, 2024 at 07:03:35PM +0000, Colton Lewis wrote:
>> Break the assignment logic for misc flags into their own respective
>> functions to reduce the complexity of the nested logic.

>> Signed-off-by: Colton Lewis <coltonlewis@...gle.com>
>> Reviewed-by: Oliver Upton <oliver.upton@...ux.dev>
>> ---
>>   arch/x86/events/core.c            | 32 +++++++++++++++++++++++--------
>>   arch/x86/include/asm/perf_event.h |  2 ++
>>   2 files changed, 26 insertions(+), 8 deletions(-)

>> diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
>> index d19e939f3998..9fdc5fa22c66 100644
>> --- a/arch/x86/events/core.c
>> +++ b/arch/x86/events/core.c
>> @@ -3011,16 +3011,35 @@ unsigned long  
>> perf_arch_instruction_pointer(struct pt_regs *regs)
>>   	return regs->ip + code_segment_base(regs);
>>   }

>> +static unsigned long common_misc_flags(struct pt_regs *regs)
>> +{
>> +	if (regs->flags & PERF_EFLAGS_EXACT)
>> +		return PERF_RECORD_MISC_EXACT_IP;
>> +
>> +	return 0;
>> +}
>> +
>> +unsigned long perf_arch_guest_misc_flags(struct pt_regs *regs)
>> +{
>> +	unsigned long guest_state = perf_guest_state();
>> +	unsigned long flags = common_misc_flags(regs);

> This is double common_misc and makes no sense

I'm confused what you mean. Are you referring to starting with
common_misc_flags in both perf_arch_misc_flags and
perf_arch_guest_misc_flags so possibly the common_msic_flags are set
twice?

That seems like a good thing that common flags are set wherever they
apply. You can't guarantee where perf_arch_guest_misc_flags may be
called in the future.
>> +
>> +	if (!(guest_state & PERF_GUEST_ACTIVE))
>> +		return flags;
>> +
>> +	if (guest_state & PERF_GUEST_USER)
>> +		return flags & PERF_RECORD_MISC_GUEST_USER;
>> +	else
>> +		return flags & PERF_RECORD_MISC_GUEST_KERNEL;

> And this is just broken garbage, right?

>> +}

> Did you mean to write:

> unsigned long perf_arch_guest_misc_flags(struct pt_regs *regs)
> {
> 	unsigned long guest_state = perf_guest_state();
> 	unsigned long flags = 0;

> 	if (guest_state & PERF_GUEST_ACTIVE) {
> 		if (guest_state & PERF_GUEST_USER)
> 			flags |= PERF_RECORD_MISC_GUEST_USER;
> 		else
> 			flags |= PERF_RECORD_MISC_GUEST_KERNEL;
> 	}

> 	return flags;
> }

Ok, my mistake was using & instead of |, but the branches are
functionally the same.

I'll use something closer to your suggestion.

>>   unsigned long perf_arch_misc_flags(struct pt_regs *regs)
>>   {
>>   	unsigned int guest_state = perf_guest_state();
>> -	int misc = 0;
>> +	unsigned long misc = common_misc_flags(regs);

> Because here you do the common thing..


>>   	if (guest_state) {
>> -		if (guest_state & PERF_GUEST_USER)
>> -			misc |= PERF_RECORD_MISC_GUEST_USER;
>> -		else
>> -			misc |= PERF_RECORD_MISC_GUEST_KERNEL;
>> +		misc |= perf_arch_guest_misc_flags(regs);

> And here you mix in the guest things.

>>   	} else {
>>   		if (user_mode(regs))
>>   			misc |= PERF_RECORD_MISC_USER;