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Date: Tue, 12 Mar 2024 08:47:16 -0700
From: Oliver Upton <oliver.upton@linux.dev>
To: David Woodhouse <dwmw2@infradead.org>
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>,
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	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>,
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	"Rafael J. Wysocki" <rafael@kernel.org>,
	Len Brown <len.brown@intel.com>, Pavel Machek <pavel@ucw.cz>,
	David Woodhouse <dwmw@amazon.co.uk>,
	Mostafa Saleh <smostafa@google.com>,
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	linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
	kvmarm@lists.linux.dev, linux-pm@vger.kernel.org
Subject: Re: [RFC PATCH 1/2] KVM: arm64: Add PSCI SYSTEM_OFF2 function for
 hibernation
Message-ID: <ZfB5BMfLHWhQXLZY@thinky-boi>
References: <20240312135958.727765-1-dwmw2@infradead.org>
 <20240312135958.727765-2-dwmw2@infradead.org>
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In-Reply-To: <20240312135958.727765-2-dwmw2@infradead.org>

Hi,

On Tue, Mar 12, 2024 at 01:51:28PM +0000, David Woodhouse wrote:
> From: David Woodhouse <dwmw@amazon.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. There are other
features being done in userspace (e.g. vCPU hotplug) built on
intercepting hypercalls, this seems to be a reasonable candidate too.

[*] https://docs.kernel.org/virt/kvm/devices/vm.html#attribute-kvm-arm-vm-smccc-filter-w-o

-- 
Thanks,
Oliver