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Date: Mon, 05 Feb 2024 14:16:07 +0000
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From: Marc Zyngier <maz@kernel.org>
To: Oliver Upton <oliver.upton@linux.dev>
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Subject: Re: [PATCH v4 2/7] arm64: KVM: Use shared area to pass PMU event state to hypervisor
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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.