linux-kernel - [PATCH] x86/perf: Default freeze_on

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Date:   Fri, 21 Jan 2022 23:26:44 -0800
From:   Kyle Huey <me@...ehuey.com>
To:     linux-kernel@...r.kernel.org, Kan Liang <kan.liang@...ux.intel.com>
Cc:     linux-perf-users@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
        x86@...nel.org, Dave Hansen <dave.hansen@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Namhyung Kim <namhyung@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Mark Rutland <mark.rutland@....com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Robert O'Callahan <rocallahan@...il.com>,
        Keno Fischer <keno@...iacomputing.com>
Subject: [PATCH] x86/perf: Default freeze_on_smi on for Comet Lake and later.

Beginning in Comet Lake, Intel extended the concept of privilege rings to
SMM.[0] A side effect of this is that events caused by execution of code
in SMM are now visible to performance counters with IA32_PERFEVTSELx.USR
set.

rr[1] depends on exact counts of performance events for the user space
tracee, so this change in behavior is fatal for us. It is, however, easily
corrected by setting IA32_DEBUGCTL.FREEZE_WHILE_SMM to 1 (visible in sysfs
as /sys/devices/cpu/freeze_on_smi). While we can and will tell our users to
set freeze_on_smi manually when appropriate, because observing events in
SMM is rarely useful to anyone, we propose to change the default value of
this switch.

In this patch I have assumed that all non-Atom Intel microarchitectures
starting with Comet Lake behave like this but it would be good for someone
at Intel to verify that.

[0] See the Intel white paper "Trustworthy SMM on the Intel vPro Platform"
at https://bugzilla.kernel.org/attachment.cgi?id=300300, particularly the
end of page 5.

[1] https://rr-project.org/

Signed-off-by: Kyle Huey <khuey@...ehuey.com>
---
 arch/x86/events/intel/core.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
index fd9f908debe5..9604e19c8761 100644
--- a/arch/x86/events/intel/core.c
+++ b/arch/x86/events/intel/core.c
@@ -6094,6 +6094,11 @@ __init int intel_pmu_init(void)
 			x86_pmu.commit_scheduling = intel_tfa_commit_scheduling;
 		}
 
+		if (boot_cpu_data.x86_model == INTEL_FAM6_COMETLAKE_L ||
+		    boot_cpu_data.x86_model == INTEL_FAM6_COMETLAKE) {
+			x86_pmu.attr_freeze_on_smi = 1;
+		}
+
 		pr_cont("Skylake events, ");
 		name = "skylake";
 		break;
@@ -6135,6 +6140,7 @@ __init int intel_pmu_init(void)
 		x86_pmu.num_topdown_events = 4;
 		x86_pmu.update_topdown_event = icl_update_topdown_event;
 		x86_pmu.set_topdown_event_period = icl_set_topdown_event_period;
+		x86_pmu.attr_freeze_on_smi = 1;
 		pr_cont("Icelake events, ");
 		name = "icelake";
 		break;
@@ -6172,6 +6178,7 @@ __init int intel_pmu_init(void)
 		x86_pmu.num_topdown_events = 8;
 		x86_pmu.update_topdown_event = icl_update_topdown_event;
 		x86_pmu.set_topdown_event_period = icl_set_topdown_event_period;
+		x86_pmu.attr_freeze_on_smi = 1;
 		pr_cont("Sapphire Rapids events, ");
 		name = "sapphire_rapids";
 		break;
@@ -6217,6 +6224,7 @@ __init int intel_pmu_init(void)
 		 * x86_pmu.rtm_abort_event.
 		 */
 		x86_pmu.rtm_abort_event = X86_CONFIG(.event=0xc9, .umask=0x04);
+		x86_pmu.attr_freeze_on_smi = 1;
 
 		td_attr = adl_hybrid_events_attrs;
 		mem_attr = adl_hybrid_mem_attrs;
-- 
2.34.1