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

From: Will Deacon <will.deacon@arm.com>
To: Ganapatrao Kulkarni <gpkulkarni@gmail.com>
Cc: Ganapatrao Kulkarni <ganapatrao.kulkarni@cavium.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Marc Zyngier <marc.zyngier@arm.com>,
	jnair@caviumnetworks.com
Subject: Re: [PATCH] arm64: perf: Count EL2 events if either of kernel and hyp are not excluded
Date: Tue, 4 Apr 2017 13:26:32 +0100	[thread overview]
Message-ID: <20170404122632.GF14898@arm.com> (raw)
In-Reply-To: <CAFpQJXW_2=8Jwqrt4k6Em6B=TBh_XA2x23OTMbpzkwKZV2x4jA@mail.gmail.com>

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@arm.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@cavium.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.

> 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?

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

Will