Re: [PATCH v2 7/8] x86, perf: Only allow rdpmc if a perf_event is mapped

[Andy Lutomirski]

On Mon, Oct 27, 2014 at 8:35 PM, Hillf Danton <hillf.zj@xxxxxxxxxxxxxxx> wrote:
>> -----Original Message-----
>> From: Andy Lutomirski [mailto:luto@xxxxxxxxxxxxxx]
>> Sent: Monday, October 27, 2014 11:45 PM
>> To: Hillf Danton
>> Cc: Peter Zijlstra; Ingo Molnar; Vince Weaver; Paul Mackerras; Kees Cook; Arnaldo Carvalho de Melo; Andrea Arcangeli; linux-
>> kernel@xxxxxxxxxxxxxxx; Valdis Kletnieks
>> Subject: Re: [PATCH v2 7/8] x86, perf: Only allow rdpmc if a perf_event is mapped
>>
> CPU D                   CPU A
> switch_mm
> load_mm_cr4
>                         x86_pmu_event_unmapped
>
> I wonder if the X86_CR4_PCE set on CPU D is
> cleared by CPU A by broadcasting IPI.
>

It should be okay.  The IPI does:

+       if (current->mm)
+               load_mm_cr4(current->mm);

which refers to the current task running on the targetted CPU, not to
the IPI sender's task.  So, if it happens after a context switch, it
will harmlessly reload the new task's cr4.

refresh_pce can't happen in between switch_mm and updating current,
since irqs are off for the entire duration of the context switch.

--Andy
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