linux-kernel - Re: [kvm-unit-tests patch v3 3/8] x86/pmu: Fix incorrect masking of fixed counters

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Message-ID: <b6a694a0-5e6d-4cb6-947e-e3b00a4fd057@linux.intel.com>
Date: Fri, 21 Nov 2025 09:25:32 +0800
From: "Mi, Dapeng" <dapeng1.mi@...ux.intel.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, 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>,
 Xiaoyao Li <xiaoyao.li@...el.com>, Dapeng Mi <dapeng1.mi@...el.com>,
 dongsheng <dongsheng.x.zhang@...el.com>, Yi Lai <yi1.lai@...el.com>
Subject: Re: [kvm-unit-tests patch v3 3/8] x86/pmu: Fix incorrect masking of
 fixed counters


On 11/21/2025 6:28 AM, Sean Christopherson wrote:
> On Wed, Sep 03, 2025, Dapeng Mi wrote:
>> From: dongsheng <dongsheng.x.zhang@...el.com>
>>
>> The current implementation mistakenly limits the width of fixed counters
>> to the width of GP counters. Corrects the logic to ensure fixed counters
>> are properly masked according to their own width.
>>
>> Opportunistically refine the GP counter bitwidth processing code.
>>
>> Signed-off-by: dongsheng <dongsheng.x.zhang@...el.com>
>> Co-developed-by: Dapeng Mi <dapeng1.mi@...ux.intel.com>
>> Signed-off-by: Dapeng Mi <dapeng1.mi@...ux.intel.com>
>> Tested-by: Yi Lai <yi1.lai@...el.com>
>> ---
>>  x86/pmu.c | 8 +++-----
>>  1 file changed, 3 insertions(+), 5 deletions(-)
>>
>> diff --git a/x86/pmu.c b/x86/pmu.c
>> index 04946d10..44c728a5 100644
>> --- a/x86/pmu.c
>> +++ b/x86/pmu.c
>> @@ -556,18 +556,16 @@ static void check_counter_overflow(void)
>>  		int idx;
>>  
>>  		cnt.count = overflow_preset;
>> -		if (pmu_use_full_writes())
>> -			cnt.count &= (1ull << pmu.gp_counter_width) - 1;
>> -
>>  		if (i == pmu.nr_gp_counters) {
>>  			if (!pmu.is_intel)
>>  				break;
>>  
>>  			cnt.ctr = fixed_events[0].unit_sel;
>> -			cnt.count = (&cnt);
> Per commit 7ec3b67a ("x86/pmu: Reset the expected count of the fixed counter 0
> when i386"), re-measuring for the fixed counter is necessary when running a 32-bit
> guests.  I didn't see failures (spotted this by inspection), but I don't see any
> point in making this change without good reason.

Thanks. I didn't realized that the 2nd measure_for_overflow() is intended
to add ...


>
>> -			cnt.count &= (1ull << pmu.gp_counter_width) - 1;
>> +			cnt.count &= (1ull << pmu.fixed_counter_width) - 1;
>>  		} else {
>>  			cnt.ctr = MSR_GP_COUNTERx(i);
>> +			if (pmu_use_full_writes())
>> +				cnt.count &= (1ull << pmu.gp_counter_width) - 1;
>>  		}
>>  
>>  		if (i % 2)
>> -- 
>> 2.34.1
>>