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

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Message-ID: <20250425134323.GA35881@noisy.programming.kicks-ass.net>
Date: Fri, 25 Apr 2025 15:43:23 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: "Liang, Kan" <kan.liang@...ux.intel.com>
Cc: Mingwei Zhang <mizhang@...gle.com>, 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 Fri, Apr 25, 2025 at 09:06:26AM -0400, Liang, Kan wrote:
> 
> 
> 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.

But all that should remain in x86, there is no reason what so ever to
leak this into generic code.