linux-kernel - Re: [PATCH] KVM: arm64: nv: Set ISTATUS for emulated timers, If timer expired

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Message-ID: <Z6ZMdv8jJUvYwQzT@linux.dev>
Date: Fri, 7 Feb 2025 10:09:58 -0800
From: Oliver Upton <oliver.upton@...ux.dev>
To: Marc Zyngier <maz@...nel.org>
Cc: Eric Auger <eauger@...hat.com>,
	Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com>,
	kvmarm <kvmarm@...ts.linux.dev>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	christoffer.dall@....com, suzuki.poulose@....com, will@...nel.org,
	catalin.marinas@....com, coltonlewis@...gle.com, joey.gouly@....com,
	yuzenghui@...wei.com, darren@...amperecomputing.com,
	vishnu@...amperecomputing.com
Subject: Re: [PATCH] KVM: arm64: nv: Set ISTATUS for emulated timers, If
 timer expired

Hey,

On Fri, Feb 07, 2025 at 05:45:33PM +0000, Marc Zyngier wrote:
> I found at least one issue that could fail the migration. Before the
> VM starts running, we limit the feature set to the subset we actually
> support with NV.
> 
> By doing this, we also change the value of IDreg fields that are not
> writable, because they describe features that we don't support.
> Obviously, that fails on restore.
> 
> I need to have a think...

We spoke about this a while ago (and I forgot til now), but I was
wondering if we could use vCPU feature flags to describe NV, including
the selection between FEAT_E2H0 and FEAT_VHE.

I think this might match userspace expectations a bit more closely where
the state of the ID registers after init gives the actual feature set
supported by the VM.

-- 
Thanks,
Oliver