linux-kernel - Re: [RFC PATCH 06/34] KVM: gunyah: Add initial Gunyah backend support

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Message-ID: <2700e50c6ca07b58c4a4a0c219815cf3e1319f35.camel@infradead.org>
Date: Fri, 01 Aug 2025 13:25:43 +0100
From: David Woodhouse <dwmw2@...radead.org>
To: Karim Manaouil <karim.manaouil@...aro.org>,
 linux-kernel@...r.kernel.org,  kvm@...r.kernel.org,
 linux-arm-msm@...r.kernel.org,  linux-arm-kernel@...ts.infradead.org,
 kvmarm@...ts.linux.dev, Steven Price <steven.price@....com>, Gavin Shan
 <gshan@...hat.com>, Suzuki Poulose's avatarSuzuki K Poulose
 <suzuki.poulose@....com>, "Usug, Ugur" <ugurus@...zon.co.uk>
Cc: Alexander Graf <graf@...zon.com>, Alex Elder <elder@...nel.org>, Catalin
 Marinas <catalin.marinas@....com>, Fuad Tabba <tabba@...gle.com>, Joey
 Gouly <joey.gouly@....com>,  Jonathan Corbet <corbet@....net>, Marc Zyngier
 <maz@...nel.org>, Mark Brown <broonie@...nel.org>,  Mark Rutland
 <mark.rutland@....com>, Oliver Upton <oliver.upton@...ux.dev>, Paolo
 Bonzini <pbonzini@...hat.com>, Prakruthi Deepak Heragu
 <quic_pheragu@...cinc.com>,  Quentin Perret <qperret@...gle.com>, Rob
 Herring <robh@...nel.org>, Srinivas Kandagatla <srini@...nel.org>, 
 Srivatsa Vaddagiri <quic_svaddagi@...cinc.com>, Will Deacon
 <will@...nel.org>, Haripranesh S <haripran@....qualcomm.com>,  Carl van
 Schaik <cvanscha@....qualcomm.com>, Murali Nalajala <mnalajal@...cinc.com>,
 Sreenivasulu Chalamcharla <sreeniva@....qualcomm.com>, Trilok Soni
 <tsoni@...cinc.com>, Stefan Schmidt <stefan.schmidt@...aro.org>
Subject: Re: [RFC PATCH 06/34] KVM: gunyah: Add initial Gunyah backend
 support

On Thu, 2025-04-24 at 15:13 +0100, Karim Manaouil wrote:
> 
> - Introduces a new Kconfig split: `CONFIG_KVM_ARM` for native support,
>   and a variant for Gunyah-backed virtualization.
> - Adds `gunyah.c`, a new arch backend file that implements the minimal
>   KVM architecture callbacks and stub interfaces required by the KVM
>   core to build and boot.
> - Refactors Makefile and build rules to support mutually exclusive
>   builds of `CONFIG_KVM_ARM` and `CONFIG_GUNYAH`.
> - Introduces a dummy implementation of required KVM stubs such as:
>   `kvm_arch_init_vm()`, `kvm_arch_vcpu_create()`, `kvm_age_gfn()`, etc.

I quite like this, conceptually. I do think it's important for the
kernel to provide a generic virtualization API regardless of the
underlying hardware/firmware — that is, after all, what an OS kernel is
*for*. So the answer "not in KVM" is just fundamentally not realistic.

I'd like to see fewer #ifdefs though. The model we have on x86 with
static_calls for the AMD vs. Intel back ends seems to work out OK.

We ought to be able to come up with a model inspired by x86 where we
allow certain methods to be provided by one of many sets of 'lowvisor'
ops or whatever we want to call them:

 • Native KVM (at EL2)
 • pKVM
 • Gunyah
 • CCA
 • ... and potentially one or two others.

Currently we have completely separate work on some of those; can we
combine at least the basic hooks/operations and come up with something
which is minimally intrusive where each one just plugs in its own
implementation but the KVM userspace API is as unified as possible?

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