Messages in this thread | | | From | Kyle Huey <> | Date | Mon, 24 Jan 2022 18:59:58 -0800 | 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@linux.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@kylehuey.com> > > > > Patch seems sensible enough; I'll go queue it up unless Kan comes back > > with anything troublesome.
| |