linux-kernel - Re: [PATCH] arm64: perf: Count EL2 events if either of kernel and hyp are not excluded

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Date:   Wed, 5 Apr 2017 09:29:32 +0530
From:   Ganapatrao Kulkarni <gpkulkarni@...il.com>
To:     Will Deacon <will.deacon@....com>
Cc:     Ganapatrao Kulkarni <ganapatrao.kulkarni@...ium.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Marc Zyngier <marc.zyngier@....com>, jnair@...iumnetworks.com
Subject: Re: [PATCH] arm64: perf: Count EL2 events if either of kernel and hyp
 are not excluded

On Tue, Apr 4, 2017 at 5:56 PM, Will Deacon <will.deacon@....com> wrote:
> On Tue, Apr 04, 2017 at 05:37:10PM +0530, Ganapatrao Kulkarni wrote:
>> On Tue, Apr 4, 2017 at 4:48 PM, Will Deacon <will.deacon@....com> wrote:
>> > On Tue, Apr 04, 2017 at 04:10:55PM +0530, Ganapatrao Kulkarni wrote:
>> >> commit d98ecda (arm64: perf: Count EL2 events if the kernel is running in HYP)
>> >> is returning error for perf syscall with mixed attribute set for exclude_kernel
>> >> and exlude_hv.
>> >>
>> >> This change is breaking some applications (observed with hhvm) when
>> >> ran with VHE enabled. Adding change to enable EL2 event counting,
>> >> if either of or both of exclude_kernel and exlude_hv are not set.
>> >>
>> >> Signed-off-by: Ganapatrao Kulkarni <ganapatrao.kulkarni@...ium.com>
>> >> ---
>> >>  arch/arm64/kernel/perf_event.c | 19 ++++++++++++-------
>> >>  1 file changed, 12 insertions(+), 7 deletions(-)
>> >
>> > Hmm. When we have VHE, we can't distinguish between hypervisor and kernel,
>> > so this patch doesn't seem right to me. The code currently requires
>> > both exclude_kernel and exclude_hv to be clear before we enable profiling
>> > EL2, otherwise we're failing to exclude samples that were asked to be
>> > excluded.
>>
>> The application cant differentiate that kernel is running in EL2/VHE or in EL1
>> when VHE=1, is it makes sense to enable EL2 event counting when there
>> is request from application to either include kernel or hypervisor
>> event count, since both are same.
>
> You can make exactly the same argument against your proposal by saying that
> it makes sense to disable EL2 event counting when there is a request from
> an application to either exclude kernel or hypervisor event counting.

yes, the argument is equally valid on either side.

>
>> IMO, it is not appropriate to have different application behaviour
>> when kernel booted with VHE=0/1
>
> Then find another solution to that. How about a mechanism to advertise
> that exclude_hv is effectively always set if the kernel is running at EL2?

I am not sure, how we can advertise to user that kernel is running at EL2.
we may add a note to man page of perf_event_open?
"exclude_hv is always set, if host kernel and hypervisor are running
at same privilege level",

>
> That would mean that you would use exclude_kernel to determine the profiling
> controls for the host.

yes, this seems to be more appropriate.

>
> Will

thanks
Ganapat