linux-kernel - Re: [PATCH v4 10/38] perf/x86: Support switch_guest

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Message-ID: <e2f3b1d5-ed91-47a1-aead-28675bcca2c8@linux.intel.com>
Date: Fri, 25 Apr 2025 09:06:26 -0400
From: "Liang, Kan" <kan.liang@...ux.intel.com>
To: Peter Zijlstra <peterz@...radead.org>, Mingwei Zhang <mizhang@...gle.com>
Cc: Ingo Molnar <mingo@...hat.com>, Arnaldo Carvalho de Melo
 <acme@...nel.org>, Namhyung Kim <namhyung@...nel.org>,
 Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini
 <pbonzini@...hat.com>, Mark Rutland <mark.rutland@....com>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
 Adrian Hunter <adrian.hunter@...el.com>, Liang@...gle.com,
 "H. Peter Anvin" <hpa@...or.com>, linux-perf-users@...r.kernel.org,
 linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
 linux-kselftest@...r.kernel.org, Yongwei Ma <yongwei.ma@...el.com>,
 Xiong Zhang <xiong.y.zhang@...ux.intel.com>,
 Dapeng Mi <dapeng1.mi@...ux.intel.com>, Jim Mattson <jmattson@...gle.com>,
 Sandipan Das <sandipan.das@....com>, Zide Chen <zide.chen@...el.com>,
 Eranian Stephane <eranian@...gle.com>, Shukla Manali
 <Manali.Shukla@....com>, Nikunj Dadhania <nikunj.dadhania@....com>
Subject: Re: [PATCH v4 10/38] perf/x86: Support switch_guest_ctx interface

On 2025-04-25 7:15 a.m., Peter Zijlstra wrote:
> On Mon, Mar 24, 2025 at 05:30:50PM +0000, Mingwei Zhang wrote:
>> From: Kan Liang <kan.liang@...ux.intel.com>
>>
>> Implement switch_guest_ctx interface for x86 PMU, switch PMI to dedicated
>> KVM_GUEST_PMI_VECTOR at perf guest enter, and switch PMI back to
>> NMI at perf guest exit.
>>
>> Signed-off-by: Xiong Zhang <xiong.y.zhang@...ux.intel.com>
>> Signed-off-by: Kan Liang <kan.liang@...ux.intel.com>
>> Tested-by: Yongwei Ma <yongwei.ma@...el.com>
>> Signed-off-by: Mingwei Zhang <mizhang@...gle.com>
>> ---
>>  arch/x86/events/core.c | 12 ++++++++++++
>>  1 file changed, 12 insertions(+)
>>
>> diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
>> index 8f218ac0d445..28161d6ff26d 100644
>> --- a/arch/x86/events/core.c
>> +++ b/arch/x86/events/core.c
>> @@ -2677,6 +2677,16 @@ static bool x86_pmu_filter(struct pmu *pmu, int cpu)
>>  	return ret;
>>  }
>>  
>> +static void x86_pmu_switch_guest_ctx(bool enter, void *data)
>> +{
>> +	u32 guest_lvtpc = *(u32 *)data;
>> +
>> +	if (enter)
>> +		apic_write(APIC_LVTPC, guest_lvtpc);
>> +	else
>> +		apic_write(APIC_LVTPC, APIC_DM_NMI);
>> +}
> 
> This, why can't it use x86_pmu.guest_lvtpc here and call it a day? Why
> is that argument passed around through the generic code only to get back
> here?

The vector has to be from the KVM. However, the current interfaces only
support KVM read perf variables, e.g., perf_get_x86_pmu_capability and
perf_get_hw_event_config.
We need to add an new interface to allow the KVM write a perf variable,
e.g., perf_set_guest_lvtpc.

Thanks,
Kan