Subject: Re: [PATCH 0/2] KVM: arm64: Filtering PMU events

From: Marc Zyngier <[email-masked]>
Date: Sat, 15 Feb 2020 13:00:55 +0000

On Fri, 14 Feb 2020 18:36:13 +0000
Marc Zyngier <[email-masked]> wrote:

> It is at times necessary to prevent a guest from being able to sample
> certain events if multiple CPUs share resources such as a cache level.
> In this case, it would be interesting if the VMM could simply prevent
> certain events from being counted instead of simply not exposing a PMU.
> 
> Given that most events are not architected, there is no easy way
> to designate which events shouldn't be counted other than specifying
> the raw event number.
> 
> Since I have no idea whether it is better to use an event whitelist
> or blacklist, the proposed API takes a cue from the x86 version and
> allows either allowing or denying counting of ranges of events.
> The event space being pretty large (16bits on ARMv8.1), the default
> policy is set by the first filter that gets installed (default deny if
> we first allow, default allow if we first deny).
> 
> The filter state is global to the guest, despite the PMU being per
> CPU. I'm not sure whether it would be worth it making it CPU-private.
> 
> Anyway, I'd be interesting in comments on how people would use this.
> I'll try to push a patch against kvmtool that implement this shortly
> (what I have currently is a harcoded set of hacks).

I now have a small extension to kvmtool allowing a --pmu-filter option
to be passed on the command line (see [1]).

I've also pushed out an update[2] to the kernel side of things, making
the filtering of the cycle counter consistent and documenting that
neither SW_INCR nor CHAIN could be filtered with this mechanism (but
this is of course up for discussion).

Thanks,

	M.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/maz/kvmtool.git/commit/?h=pmu-filter
[2] https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git/log/?h=kvm-arm64/pmu-event-filter

-- 
Jazz is not dead. It just smells funny...

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