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Date: Mon, 05 Feb 2024 15:50:12 +0000
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From: Marc Zyngier <maz@kernel.org>
To: James Clark <james.clark@arm.com>
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Subject: Re: [PATCH v4 2/7] arm64: KVM: Use shared area to pass PMU event state to hypervisor
In-Reply-To: <c773393c-221e-edd1-00b7-0ce6a2481c15@arm.com>
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On Mon, 05 Feb 2024 15:37:34 +0000,
James Clark <james.clark@arm.com> wrote:
> 
> 
> 
> On 05/02/2024 14:52, Marc Zyngier wrote:
> > On Mon, 05 Feb 2024 14:17:10 +0000,
> > James Clark <james.clark@arm.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@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.
> >>>
> >>
> >> 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.
> > 
> 
> Hmmm in that case if there's currently no way to distinguish between
> normal VMs and pVMs in protected-mode then what I was thinking of
> probably won't work.

Have you looked? kvm_vm_is_protected() has been in for a while, even
if that's not a lot. The upcoming code will flesh this helper out,

	M.

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