linux-kernel - Re: [kvm-unit-tests patch 1/5] x86/pmu: Add helper to detect Intel overcount issues

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Message-ID: <449b2fa5-775b-41ab-8a9e-5a43e855e931@linux.intel.com>
Date: Wed, 16 Jul 2025 09:13:36 +0800
From: "Mi, Dapeng" <dapeng1.mi@...ux.intel.com>
To: Xiaoyao Li <xiaoyao.li@...el.com>, Sean Christopherson
 <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 Jim Mattson <jmattson@...gle.com>, Mingwei Zhang <mizhang@...gle.com>,
 Zide Chen <zide.chen@...el.com>, Das Sandipan <Sandipan.Das@....com>,
 Shukla Manali <Manali.Shukla@....com>, Yi Lai <yi1.lai@...el.com>,
 Dapeng Mi <dapeng1.mi@...el.com>, dongsheng <dongsheng.x.zhang@...el.com>
Subject: Re: [kvm-unit-tests patch 1/5] x86/pmu: Add helper to detect Intel
 overcount issues


On 7/15/2025 9:27 PM, Xiaoyao Li wrote:
> On 7/13/2025 1:49 AM, Dapeng Mi wrote:
>> From: dongsheng <dongsheng.x.zhang@...el.com>
>>
>> For Intel Atom CPUs, the PMU events "Instruction Retired" or
>> "Branch Instruction Retired" may be overcounted for some certain
>> instructions, like FAR CALL/JMP, RETF, IRET, VMENTRY/VMEXIT/VMPTRLD
>> and complex SGX/SMX/CSTATE instructions/flows.
>>
>> The detailed information can be found in the errata (section SRF7):
>> https://edc.intel.com/content/www/us/en/design/products-and-solutions/processors-and-chipsets/sierra-forest/xeon-6700-series-processor-with-e-cores-specification-update/errata-details/
>>
>> For the Atom platforms before Sierra Forest (including Sierra Forest),
>> Both 2 events "Instruction Retired" and "Branch Instruction Retired" would
>> be overcounted on these certain instructions, but for Clearwater Forest
>> only "Instruction Retired" event is overcounted on these instructions.
>>
>> So add a helper detect_inst_overcount_flags() to detect whether the
>> platform has the overcount issue and the later patches would relax the
>> precise count check by leveraging the gotten overcount flags from this
>> helper.
>>
>> Signed-off-by: dongsheng <dongsheng.x.zhang@...el.com>
>> [Rewrite comments and commit message - Dapeng]
>> Signed-off-by: Dapeng Mi <dapeng1.mi@...ux.intel.com>
>> Tested-by: Yi Lai <yi1.lai@...el.com>
>> ---
>>   lib/x86/processor.h | 17 ++++++++++++++++
>>   x86/pmu.c           | 47 +++++++++++++++++++++++++++++++++++++++++++++
>>   2 files changed, 64 insertions(+)
>>
>> diff --git a/lib/x86/processor.h b/lib/x86/processor.h
>> index 62f3d578..3f475c21 100644
>> --- a/lib/x86/processor.h
>> +++ b/lib/x86/processor.h
>> @@ -1188,4 +1188,21 @@ static inline bool is_lam_u57_enabled(void)
>>   	return !!(read_cr3() & X86_CR3_LAM_U57);
>>   }
>>   
>> +static inline u32 x86_family(u32 eax)
>> +{
>> +	u32 x86;
>> +
>> +	x86 = (eax >> 8) & 0xf;
>> +
>> +	if (x86 == 0xf)
>> +		x86 += (eax >> 20) & 0xff;
>> +
>> +	return x86;
>> +}
>> +
>> +static inline u32 x86_model(u32 eax)
>> +{
>> +	return ((eax >> 12) & 0xf0) | ((eax >> 4) & 0x0f);
>> +}
> It seems to copy the implementation of kvm selftest.
>
> I need to point it out that it's not correct (because I fixed the 
> similar issue on QEMU recently).
>
> We cannot count Extended Model ID unconditionally. Intel counts Extended 
> Model when (base) Family is 0x6 or 0xF, while AMD counts EXtended Model 
> when (base) Family is 0xF.
>
> You can refer to kernel's x86_model() in arch/x86/lib/cpu.c, while it 
> optimizes the condition to "family >= 0x6", which seems to have the 
> assumption that Intel doesn't have processor with family ID from 7 to 
> 0xe and AMD doesn't have processor with family ID from 6 to 0xe.

Sure. Thanks for reviewing.