Re: [PATCH v2] arm64: perf: Use only exclude_kernel attribute when kernel is running in HYP

[Jayachandran C]

On Fri, Apr 28, 2017 at 05:38:23PM +0100, Will Deacon wrote:
> Hi guys,
> 
> On Fri, Apr 28, 2017 at 01:46:24PM +0000, Jayachandran C wrote:
> > On Thu, Apr 27, 2017 at 06:37:59PM +0100, Will Deacon wrote:
> > > > If my understanding is correct, the sysfs suggestion above is going to
> > > > add API complexity without solving the issue. Ignoring the exclude_hv if
> > > > it cannot be honored would be a better solution.
> > > 
> > > Better for HHVM, sure, but I don't think it's better in general. It means
> > > that we silently do the opposite of what the user has requested in some
> > > configurations.
> > 
> > If my understanding is correct, when is_kernel_in_hyp_mode() is true,
> > the kernel is in EL2 and there is no real hypervisor with hvc calls
> > from kernel. Ignoring the exclude_hv would be correct.
> > 
> > When kernel is in EL1, it would be correct to consider exclude_hv to
> > skip events in EL2 (reached with hvc).
> > 
> > I don't see the issue, can you please give more detail on the config
> > with unexpected behavior?
> 
> This got me thinking, so I tried to look at the history of exclude_hv. It
> turns out it was added in 0475f9ea8e2c ("perf_counters: allow users to
> count user, kernel and/or hypervisor events") for PowerPC, not x86 (where
> this doesn't seem to be supported).
> 
> Notably, it looks like it's always ignored for the x86 CPU PMU, and ignored
> on PowerPC when a hypervisor is not present. I think that backs up your
> suggestion that we should ignore it when is_kernel_in_hyp_mode() is true.
> 
> In which case, I withdraw my objection to ignoring exclude_hv when running
> in hyp mode, but please add a comment explaining the rationale!

Thanks, we will send out an updated patch with a commit message summarizing
this disucssion.

JC