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

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Message-ID: <20170428163823.GX13675@arm.com>
Date:   Fri, 28 Apr 2017 17:38:23 +0100
From:   Will Deacon <will.deacon@....com>
To:     Jayachandran C <jnair@...iumnetworks.com>
Cc:     "Pinski, Andrew" <Andrew.Pinski@...ium.com>,
        "Jayachandran C." <c.jayachandran@...il.com>,
        Ganapatrao Kulkarni <gpkulkarni@...il.com>,
        Mark Rutland <mark.rutland@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "acme@...nel.org" <acme@...nel.org>,
        "alexander.shishkin@...ux.intel.com" 
        <alexander.shishkin@...ux.intel.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        "Nair, Jayachandran" <Jayachandran.Nair@...ium.com>,
        "Kulkarni, Ganapatrao" <Ganapatrao.Kulkarni@...ium.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2] arm64: perf: Use only exclude_kernel attribute when
 kernel is running in HYP

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!

Will