Messages in this thread | | | Date | Mon, 24 Jan 2022 13:21:51 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH] x86/perf: Default freeze_on_smi on for Comet Lake and later. |
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On Fri, Jan 21, 2022 at 11:26:44PM -0800, Kyle Huey wrote: > 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. >
Kan, can you look at 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@kylehuey.com>
Patch seems sensible enough; I'll go queue it up unless Kan comes back with anything troublesome.
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