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

[Will Deacon]

On Wed, Apr 05, 2017 at 09:29:32AM +0530, Ganapatrao Kulkarni wrote:
> On Tue, Apr 4, 2017 at 5:56 PM, Will Deacon <will.deacon@xxxxxxx> 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@xxxxxxx> 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@xxxxxxxxxx>
> >> >> ---
> >> >>  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",

I was thinking of putting something into sysfs, alongside the other things
we have in there. For example, a file that describes whether any of the
perf_event_attr behave as though they are fixed to a certain value. We
should involve the perf maintainers (and perf tool developers) in this,
but perhaps something like an attr directory, where we could have a file
called exclude_hv that contains the value 1.

Will