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

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Date: Mon, 05 Feb 2024 14:52:29 +0000
From: Marc Zyngier <maz@...nel.org>
To: James Clark <james.clark@....com>
Cc: Oliver Upton <oliver.upton@...ux.dev>,
	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 14:17:10 +0000,
James Clark <james.clark@....com> wrote:
>
> On 05/02/2024 13:21, Oliver Upton 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.
> > 
> 
> In that case I can hide all this behind CONFIG_NVHE_EL2_DEBUG for pKVM.
> This will also have the effect of disabling PMU again for pKVM because I
> moved that into this new shared area.

I'm not sure what you have in mind, but dropping PMU support for
non-protected guests when protected-mode is enabled is not an
acceptable outcome.

Hiding the trace behind a debug option is fine as this is a global
setting that has no userspace impact, but impacting guests isn't.

	M.

-- 
Without deviation from the norm, progress is not possible.