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: <865xz36lp4.wl-maz@kernel.org>
Date: Mon, 05 Feb 2024 14:16:07 +0000
From: Marc Zyngier <maz@...nel.org>
To: Oliver Upton <oliver.upton@...ux.dev>
Cc: James Clark <james.clark@....com>,
	coresight@...ts.linaro.org,
	linux-arm-kernel@...ts.infradead.org,
	kvmarm@...ts.linux.dev,
	broonie@...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, 05 Feb 2024 13:21:26 +0000,
Oliver Upton <oliver.upton@...ux.dev> wrote:
> 
> On Mon, Feb 05, 2024 at 01:15:36PM +0000, Marc Zyngier wrote:
> > On Mon, 05 Feb 2024 13:04:51 +0000,
> > Oliver Upton <oliver.upton@...ux.dev> wrote:
> > > 
> > > 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.
> > 
> > Limiting tracing to non-protected mode is a must IMO. Allowing tracing
> > when pKVM is enabled is a sure way to expose secrets that should
> > stay... secret. The only exception I can think of is when
> > CONFIG_NVHE_EL2_DEBUG is enabled, at which point all bets are off.
> 
> Zero argument there :) I left off the "and PMU" part of what I was
> saying, because that was a feature that semi-worked in protected mode
> before VM/VCPU shadowing support landed.

Indeed. The goal is that as far as userspace is concerned, the host
running in protected mode shouldn't impair the ability to run
non-protected VMs, and it should all be hunky-dory, unless you
explicitly ask for a protected guest (at which point you are facing a
lot of restrictions).

PMU definitely falls into that last bucket, although I would hope that
we eventually get some support by context-switching the whole of the
PMU state. Don't worry, it's going to be cheap...

Thanks,

	M.


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Without deviation from the norm, progress is not possible.