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From: Peter Zijlstra <peterz@infradead.org>
To: "Liang, Kan" <kan.liang@linux.intel.com>
Cc: Kyle Huey <me@kylehuey.com>,
	linux-kernel@vger.kernel.org, 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>,
	Andi Kleen <ak@linux.intel.com>
Subject: Re: [PATCH] x86/perf: Default freeze_on_smi on for Comet Lake and later.
Date: Mon, 24 Jan 2022 17:30:04 +0100	[thread overview]
Message-ID: <Ye7UDFAV5/OPaE5N@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <3c35dc76-c187-8d3f-7fc9-75de32e7cbf6@linux.intel.com>

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.

  reply	other threads:[~2022-01-24 16:30 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-01-22  7:26 [PATCH] x86/perf: Default freeze_on_smi on for Comet Lake and later Kyle Huey
2022-01-24 12:21 ` Peter Zijlstra
2022-01-24 16:00   ` Liang, Kan
2022-01-24 16:30     ` Peter Zijlstra [this message]
2022-01-24 17:03       ` Liang, Kan
2022-01-24 17:16         ` Peter Zijlstra
2022-01-25  2:59     ` Kyle Huey
2022-01-25 13:57       ` Liang, Kan
2022-01-26 14:04         ` Peter Zijlstra
2022-01-26 14:58           ` Liang, Kan
2022-01-27  2:22 ` Andrew Cooper
2022-01-27 11:31   ` Peter Zijlstra
2022-01-27 11:56     ` Andrew Cooper
2022-02-03 14:33     ` [tip: perf/urgent] x86/perf: Default set FREEZE_ON_SMI for all tip-bot2 for Peter Zijlstra

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