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

[Peter Zijlstra]

On Mon, Jan 24, 2022 at 11:00:56AM -0500, Liang, Kan wrote:
> 
> 
> On 1/24/2022 7:21 AM, Peter Zijlstra wrote:
> > 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.
> 
> + Andi
> 
> From we heard many times from sophisticated customers, they really hate
> blind spots. They want to see everything. That's why we set freeze_on_smi to
> 0 as default. I think the patch breaks the principle.

Well, USR really, as in *REALLY* should not be counting SMM. That's just
plain broken.

There's maybe an argument to include it in OS, but USR is ring-3.