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

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Message-ID: <d140bb65f02d2b0e33d176beef400d9e22bebafa.camel@infradead.org>
Date: Wed, 13 Mar 2024 12:53:45 +0000
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 Tue, 2024-03-12 at 08:47 -0700, Oliver Upton wrote:
> Hi,
> 
> On Tue, Mar 12, 2024 at 01:51:28PM +0000, David Woodhouse wrote:
> > From: David Woodhouse <dwmw@...zon.co.uk>
> > 
> > The PSCI v1.3 specification (alpha) adds support for a SYSTEM_OFF2 function
> > which is analogous to ACPI S4 state. This will allow hosting environments
> > to determine that a guest is hibernated rather than just powered off, and
> > ensure that they preserve the virtual environment appropriately to allow
> > the guest to resume safely (or bump the hardware_signature in the FACS to
> > trigger a clean reboot instead).
> > 
> > The beta version will be changed to say that PSCI_FEATURES returns a bit
> > mask of the supported hibernate types, which is implemented here.
> 
> Have you considered doing the PSCI implementation in userspace? The
> SMCCC filter [*] was added for this exact purpose. 
> 

For the purpose of deprecating the in-KVM PSCI implementation and
reimplementing it in VMMs? So we're never going to add new features and
versions to the kernel PSCI?

If that's the case then I suppose I can send the patch to clearly
document the in-KVM PSCI as deprecated, and do it that way.

But to answer your question directly, no I hadn't considered that. I
was just following the existing precedent of adding new optional PSCI
features like SYSTEM_RESET2.

I didn't think that the addition of SYSTEM_OFF2 in precisely the same
fashion would be the straw that broke the camel's back, and pushed us
to reimplement it in userspace instead.

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