linux-kernel - Re: [RFC PATCH 1/2] KVM: arm64: Add PSCI SYSTEM

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Date: Thu, 14 Mar 2024 00:01:45 +0100
From: David Woodhouse <dwmw2@...radead.org>
To: Oliver Upton <oliver.upton@...ux.dev>
Cc: linux-arm-kernel@...ts.infradead.org, kvm@...r.kernel.org, Paolo Bonzini
 <pbonzini@...hat.com>, Jonathan Corbet <corbet@....net>, Marc Zyngier
 <maz@...nel.org>, James Morse <james.morse@....com>, Suzuki K Poulose
 <suzuki.poulose@....com>, Zenghui Yu <yuzenghui@...wei.com>, Catalin
 Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, Mark
 Rutland <mark.rutland@....com>, Lorenzo Pieralisi <lpieralisi@...nel.org>,
 "Rafael J. Wysocki" <rafael@...nel.org>, Len Brown <len.brown@...el.com>,
 Pavel Machek <pavel@....cz>, Mostafa Saleh <smostafa@...gle.com>,
 Jean-Philippe Brucker <jean-philippe@...aro.org>, 
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
 kvmarm@...ts.linux.dev,  linux-pm@...r.kernel.org
Subject: Re: [RFC PATCH 1/2] KVM: arm64: Add PSCI SYSTEM_OFF2 function for
 hibernation

On Wed, 2024-03-13 at 12:42 -0700, Oliver Upton wrote:
> I do not believe using the SMCCC filter to take SYSTEM_OFF2 to userspace
> would necessitate a _full_ userspace reimplementation. You're free to
> leave the default handler in place for functions you don't care about.
> Forwarding PSCI_VERSION, PSCI_FEATURES, and SYSTEM_OFF2 would be sufficient
> to get this off the ground, and the VMM can still advertise the rest of
> the hypercalls implemented by KVM.

Right... so we'd intercept PSCI_FEATURES *just* to indicate support for
the one call we implement in userspace, and pass all other
PSCI_FEATURES calls through to the kernel to handle the others? 

And then we'd implement SYSTEM_OFF2, hooking it up to do whatever our
existing KVM_EXIT_SYSTEM_EVENT handler *already* does for a standard
power-off, just with the extra flag to show it's a hibernate.

This concept does not fill me with joy.

> That might get you where you want to go a bit faster, since it'd avoid
> any concerns about implementing a draft ABI in the kernel.

I'd be more concerned about supporting a draft ABI in a public cloud
provider, TBH. Having it as just one more downstream kernel patch,
posted upstream but not yet merged before the final publication of the
spec, would be the least of my worries.

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