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

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Date:   Mon, 24 Jan 2022 13:21:51 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kyle Huey <me@...ehuey.com>
Cc:     linux-kernel@...r.kernel.org,
        Kan Liang <kan.liang@...ux.intel.com>,
        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>,
        Robert O'Callahan <rocallahan@...il.com>,
        Keno Fischer <keno@...iacomputing.com>
Subject: Re: [PATCH] x86/perf: Default freeze_on_smi on for Comet Lake and
 later.

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@...ehuey.com>

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