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

[Peter Zijlstra]

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@xxxxxxxxxxxx>

Patch seems sensible enough; I'll go queue it up unless Kan comes back
with anything troublesome.