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Message-ID: <d140bb65f02d2b0e33d176beef400d9e22bebafa.camel@infradead.org>
Subject: Re: [RFC PATCH 1/2] KVM: arm64: Add PSCI SYSTEM_OFF2 function for
 hibernation
From: David Woodhouse <dwmw2@infradead.org>
To: Oliver Upton <oliver.upton@linux.dev>
Cc: linux-arm-kernel@lists.infradead.org, kvm@vger.kernel.org, Paolo Bonzini
 <pbonzini@redhat.com>, Jonathan Corbet <corbet@lwn.net>, Marc Zyngier
 <maz@kernel.org>, James Morse <james.morse@arm.com>, Suzuki K Poulose
 <suzuki.poulose@arm.com>, Zenghui Yu <yuzenghui@huawei.com>, Catalin
 Marinas <catalin.marinas@arm.com>, Will Deacon <will@kernel.org>, Mark
 Rutland <mark.rutland@arm.com>, Lorenzo Pieralisi <lpieralisi@kernel.org>,
 "Rafael J. Wysocki" <rafael@kernel.org>, Len Brown <len.brown@intel.com>,
 Pavel Machek <pavel@ucw.cz>, Mostafa Saleh <smostafa@google.com>,
 Jean-Philippe Brucker <jean-philippe@linaro.org>, 
 linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
 kvmarm@lists.linux.dev,  linux-pm@vger.kernel.org
Date: Wed, 13 Mar 2024 12:53:45 +0000
In-Reply-To: <ZfB5BMfLHWhQXLZY@thinky-boi>
References: <20240312135958.727765-1-dwmw2@infradead.org>
	 <20240312135958.727765-2-dwmw2@infradead.org> <ZfB5BMfLHWhQXLZY@thinky-boi>
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On Tue, 2024-03-12 at 08:47 -0700, Oliver Upton wrote:
> Hi,
>=20
> On Tue, Mar 12, 2024 at 01:51:28PM +0000, David Woodhouse wrote:
> > From: David Woodhouse <dwmw@amazon.co.uk>
> >=20
> > The PSCI v1.3 specification (alpha) adds support for a SYSTEM_OFF2 func=
tion
> > which is analogous to ACPI S4 state. This will allow hosting environmen=
ts
> > to determine that a guest is hibernated rather than just powered off, a=
nd
> > ensure that they preserve the virtual environment appropriately to allo=
w
> > the guest to resume safely (or bump the hardware_signature in the FACS =
to
> > trigger a clean reboot instead).
> >=20
> > The beta version will be changed to say that PSCI_FEATURES returns a bi=
t
> > mask of the supported hibernate types, which is implemented here.
>=20
> Have you considered doing the PSCI implementation in userspace? The
> SMCCC filter [*] was added for this exact purpose.=C2=A0
>=20

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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