linux-kernel - Re: [PATCH v4 2/7] arm64: KVM: Use shared area to pass PMU event state to hypervisor

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Message-ID: <ZcDc8-FQo8wKavA4@linux.dev>
Date: Mon, 5 Feb 2024 13:04:51 +0000
From: Oliver Upton <oliver.upton@...ux.dev>
To: James Clark <james.clark@....com>
Cc: coresight@...ts.linaro.org, linux-arm-kernel@...ts.infradead.org,
	kvmarm@...ts.linux.dev, broonie@...nel.org, maz@...nel.org,
	suzuki.poulose@....com, acme@...nel.org,
	James Morse <james.morse@....com>,
	Zenghui Yu <yuzenghui@...wei.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>, Mike Leach <mike.leach@...aro.org>,
	Leo Yan <leo.yan@...aro.org>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Anshuman Khandual <anshuman.khandual@....com>,
	Rob Herring <robh@...nel.org>, Miguel Luis <miguel.luis@...cle.com>,
	Jintack Lim <jintack.lim@...aro.org>,
	Ard Biesheuvel <ardb@...nel.org>,
	Mark Rutland <mark.rutland@....com>, Arnd Bergmann <arnd@...db.de>,
	Vincent Donnefort <vdonnefort@...gle.com>,
	Kristina Martsenko <kristina.martsenko@....com>,
	Fuad Tabba <tabba@...gle.com>, Joey Gouly <joey.gouly@....com>,
	Akihiko Odaki <akihiko.odaki@...nix.com>,
	Jing Zhang <jingzhangos@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/7] arm64: KVM: Use shared area to pass PMU event
 state to hypervisor

On Mon, Feb 05, 2024 at 12:16:53PM +0000, James Clark wrote:
> > This now allows the host to program event counters for a protected
> > guest. That _might_ be a useful feature behind some debug option, but is
> > most definitely *not* something we want to do for pVMs generally.
> 
> Unless I'm missing something, using PMUs on protected guests was added
> by 722625c6f4c5b ("KVM: arm64: Reenable pmu in Protected Mode"). This
> change is just a refactor that will allow us to add the same behavior
> for a similar feature (tracing) without adding yet another copy of some
> state before the guest switch.

Ha, I had forgotten about that patch (and I had reviewed it!)

My interpretation of the intent for that change was to enable the usage
of vPMU for non-protected VMs. The situation has changed since then, as
we use the shadow state for vCPUs unconditionally in protected mode as
of commit  be66e67f1750 ("KVM: arm64: Use the pKVM hyp vCPU structure
in handle___kvm_vcpu_run()")

Protected mode is well understood at this point to be a WIP feature, and
that not all things are expected to work with it. Eventually we will
need a way to distinguish between 'normal' VMs and true pVMs (i.e. the
VMM selected full isolation) in nVHE, but right now what we have enables
testing of some isolation features.

> > I'm perfectly happy leaving these sorts of features broken for pKVM and
> > using the 'normal' way of getting percpu data to the nVHE hypervisor
> > otherwise.
> > 
> 
> I can do that. But do I also disable PMU at the same time in a new
> commit? Now that both PMU and tracing is working maybe it would be a
> waste to throw that away and hiding it behind an option is better. Or I
> can leave the PMU as it is and just keep tracing disabled in pKVM.
> 
> I don't mind either way, my main goal was to get exclude/include guest
> tracing working for normal VMs. For pKVM I don't have a strong opinion.

Unless someone has strong opinions about making this work in protected
mode, I am happy to see tracing support limited to the 'normal' nVHE
configuration. The protected feature as a whole is just baggage until
upstream support is completed.

-- 
Thanks,
Oliver