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, 5 Feb 2024 14:17:10 +0000
From: James Clark <james.clark@....com>
To: Oliver Upton <oliver.upton@...ux.dev>, Marc Zyngier <maz@...nel.org>
Cc: 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 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.

The same place will be used to store the state for normal nVHE and at
least then there is some code re-use and flexibility to use trace and
PMU for debugging if needed. And the copy on every switch gets deleted.