linux-kernel - Re: [Patch v2 7/7] perf/x86/intel: Add support for rdpmc user disable feature

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Message-ID: <20260113105130.GW830755@noisy.programming.kicks-ass.net>
Date: Tue, 13 Jan 2026 11:51:30 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: "Mi, Dapeng" <dapeng1.mi@...ux.intel.com>
Cc: Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>, Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Eranian Stephane <eranian@...gle.com>, linux-kernel@...r.kernel.org,
	linux-perf-users@...r.kernel.org, Dapeng Mi <dapeng1.mi@...el.com>,
	Zide Chen <zide.chen@...el.com>,
	Falcon Thomas <thomas.falcon@...el.com>,
	Xudong Hao <xudong.hao@...el.com>
Subject: Re: [Patch v2 7/7] perf/x86/intel: Add support for rdpmc user
 disable feature

On Tue, Jan 13, 2026 at 10:29:38AM +0800, Mi, Dapeng wrote:
> 
> On 1/12/2026 6:57 PM, Peter Zijlstra wrote:
> > On Mon, Jan 12, 2026 at 01:16:49PM +0800, Dapeng Mi wrote:
> >
> >> +static void intel_pmu_update_rdpmc_user_disable(struct perf_event *event)
> >> +{
> >> +	/*
> >> +	 * Counter scope's user-space rdpmc is disabled by default
> >> +	 * except two cases.
> >> +	 * a. rdpmc = 2 (user space rdpmc enabled unconditionally)
> >> +	 * b. rdpmc = 1 and the event is not a system-wide event.
> >> +	 *    The count of non-system-wide events would be cleared when
> >> +	 *    context switches, so no count data is leaked.
> >> +	 */
> >> +	if (x86_pmu_has_rdpmc_user_disable(event->pmu)) {
> >> +		if (x86_pmu.attr_rdpmc == X86_USER_RDPMC_ALWAYS_ENABLE ||
> >> +		    (x86_pmu.attr_rdpmc == X86_USER_RDPMC_CONDITIONAL_ENABLE &&
> >> +		     event->ctx->task))
> >> +			event->hw.config &= ~ARCH_PERFMON_EVENTSEL_RDPMC_USER_DISABLE;
> >> +		else
> >> +			event->hw.config |= ARCH_PERFMON_EVENTSEL_RDPMC_USER_DISABLE;
> >> +	}
> >> +}
> > Is it not simpler to invert that condition?
> >
> > 	if (x86_pmu.attr_rdpmc == X86_USER_RDPMC_ALWAYS_NEVER ||
> > 	    !event->ctx->task)
> > 		event->hw.config |= ARCH_PERFMON_EVENTSEL_RDPMC_USER_DISABLE;
> > 	else
> > 		event->hw.config &= ~ARCH_PERFMON_EVENTSEL_RDPMC_USER_DISABLE;
> 
> Hmm, it seems the logic changes along with the code change. The system-wide
> event should be allowed to read with rdpmc from user space when attr_rdpmc
> == X86_USER_RDPMC_ALWAYS_ENABLE, but it doesn't with the new code.

Bah, indeed. Oh well.