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

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Date:   Mon, 24 Jan 2022 18:59:58 -0800
From:   Kyle Huey <me@...ehuey.com>
To:     "Liang, Kan" <kan.liang@...ux.intel.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        open list <linux-kernel@...r.kernel.org>,
        linux-perf-users@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <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>,
        Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH] x86/perf: Default freeze_on_smi on for Comet Lake and later.

On Mon, Jan 24, 2022 at 8:01 AM Liang, Kan <kan.liang@...ux.intel.com> 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.

The default kernel settings for perf events prioritize preventing
information leaks to less privileged code. perf_event_paranoid
defaults to 2, preventing unprivileged users from observing kernel
space. If "sophisticated customers" want to see everything they have
already needed privileges (or an explicit opt-in through decreasing
perf_event_paranoid) for some time.

The current situation on Comet Lake+ where an unprivileged user
*cannot* observe kernel code due to security concerns but
simultaneously *must* observe SMM code seems rather absurd.

> I don't think there is a way to notify all the users that the default
> kernel value will be changed. (Yes, the end user can always check the
> /sys/devices/cpu/freeze_on_smi to get the latest value. But in practice,
> no one checks it unless some errors found.) I think it may bring
> troubles to the users if they rely on the counts in SMM.

Unfortunately the new hardware has already changed the behavior
without notifying users, no matter what we do here.

> The patch only changes the default values for some platforms, not all
> platforms. The default value is not consistent among platforms anymore.
> It can bring confusion.

I don't personally object to changing freeze_on_smi for all platforms
:) I was merely trying to limit the changes.

> All in all, we have already exposed an interface for the end-users to
> change the value. If some apps, e.g., rr, doesn't want the default
> value, I think they can always change it in the app for all platforms.
> We should still keep the freeze_on_smi to 0 as default, which should
> benefit more users.

I think "people who want to just do userspace profiling like they did
before can just change the value" is an unsatisfying answer,
especially because freeze_on_smi requires root to change.


- Kyle

>
> >>
> >> 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?
> >
>
> I'm asking internally.
>
> Thanks,
> Kan
>
> >> [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.