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

From: Marc Zyngier <maz@kernel.org>
To: Oliver Upton <oliver.upton@linux.dev>
Cc: James Clark <james.clark@arm.com>,
	coresight@lists.linaro.org, linux-arm-kernel@lists.infradead.org,
	kvmarm@lists.linux.dev, broonie@kernel.org,
	suzuki.poulose@arm.com, acme@kernel.org,
	James Morse <james.morse@arm.com>,
	Zenghui Yu <yuzenghui@huawei.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>, Mike Leach <mike.leach@linaro.org>,
	Leo Yan <leo.yan@linaro.org>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Anshuman Khandual <anshuman.khandual@arm.com>,
	Rob Herring <robh@kernel.org>,
	Miguel Luis <miguel.luis@oracle.com>,
	Jintack Lim <jintack.lim@linaro.org>,
	Ard Biesheuvel <ardb@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Vincent Donnefort <vdonnefort@google.com>,
	Kristina Martsenko <kristina.martsenko@arm.com>,
	Fuad Tabba <tabba@google.com>, Joey Gouly <joey.gouly@arm.com>,
	Akihiko Odaki <akihiko.odaki@daynix.com>,
	Jing Zhang <jingzhangos@google.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 2/7] arm64: KVM: Use shared area to pass PMU event state to hypervisor
Date: Mon, 05 Feb 2024 14:16:07 +0000	[thread overview]
Message-ID: <865xz36lp4.wl-maz@kernel.org> (raw)
In-Reply-To: <ZcDg1sP3EYZG-i_3@linux.dev>

On Mon, 05 Feb 2024 13:21:26 +0000,
Oliver Upton <oliver.upton@linux.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@linux.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.

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

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