Re: [PATCH] x86/perf: Default freeze_on_smi on for Comet Lake and later.

[Peter Zijlstra]

On Thu, Jan 27, 2022 at 02:22:23AM +0000, Andrew Cooper wrote:

> Frankly, it is an error that FREEZE_WHILE_SMM is under the kernels
> control, and not SMM's control.  After all, it's SMM handling all the
> UEFI secrets/etc.
> 
> Linux ought to set FREEZE_WHILE_SMM unilaterally, because most kernel
> profiling probably won't want interference from SMM.  Root can always
> disable FREEZE_WHILE_SMM if profiling is really wanted.
> 
> I'm not sure if anything can be done on pre-FREEZE_WHILE_SMM CPUs.  Nor
> AMD CPUs which are also gaining CPL3 SMM logic, and don't appear to have
> any equivalent functionality.

Which suggests something like this?

---
diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
index c91434056c29..5874fa088630 100644
--- a/arch/x86/events/intel/core.c
+++ b/arch/x86/events/intel/core.c
@@ -4703,6 +4703,19 @@ static __initconst const struct x86_pmu intel_pmu = {
 	.lbr_read		= intel_pmu_lbr_read_64,
 	.lbr_save		= intel_pmu_lbr_save,
 	.lbr_restore		= intel_pmu_lbr_restore,
+
+	/*
+	 * SMM has access to all 4 rings and while traditionally SMM code only
+	 * ran in CPL0, newer firmware is starting to make use of CPL3 in SMM.
+	 *
+	 * Since the EVENTSEL.{USR,OS} CPL filtering makes no distinction
+	 * between SMM or not, this results in what should be pure userspace
+	 * counters including SMM data.
+	 *
+	 * This is a clear privilege issue, therefore globally disable
+	 * counting SMM by default.
+	 */
+	.attr_freeze_on_smi	= 1,
 };
 
 static __init void intel_clovertown_quirk(void)