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Date:   Mon, 24 Jan 2022 13:21:51 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Kyle Huey <me@kylehuey.com>
Cc:     linux-kernel@vger.kernel.org,
        Kan Liang <kan.liang@linux.intel.com>,
        linux-perf-users@vger.kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        x86@kernel.org, Dave Hansen <dave.hansen@linux.intel.com>,
        Borislav Petkov <bp@alien8.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Namhyung Kim <namhyung@kernel.org>,
        Jiri Olsa <jolsa@redhat.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        Robert O'Callahan <rocallahan@gmail.com>,
        Keno Fischer <keno@juliacomputing.com>
Subject: Re: [PATCH] x86/perf: Default freeze_on_smi on for Comet Lake and
 later.
Message-ID: <Ye6Z31keWVPrsNWU@hirez.programming.kicks-ass.net>
References: <20220122072644.92292-1-khuey@kylehuey.com>
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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.