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Date: Mon, 18 Mar 2024 16:57:02 +0000
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From: Marc Zyngier <maz@kernel.org>
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>,
	Oliver Upton <oliver.upton@linux.dev>,
	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>,
	David Woodhouse <dwmw@amazon.co.uk>,
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	kvmarm@lists.linux.dev,
	linux-pm@vger.kernel.org
Subject: Re: [RFC PATCH v2 0/4] arm64: Add PSCI v1.3 SYSTEM_OFF2 support for hibernation
In-Reply-To: <20240318164646.1010092-1-dwmw2@infradead.org>
References: <20240318164646.1010092-1-dwmw2@infradead.org>
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On Mon, 18 Mar 2024 16:14:22 +0000,
David Woodhouse <dwmw2@infradead.org> wrote:
> 
> The PSCI v1.3 spec (https://developer.arm.com/documentation/den0022, 
> currently in Alpha state, hence 'RFC') adds support for a SYSTEM_OFF2 
> function enabling a HIBERNATE_OFF state which is analogous to ACPI S4. 
> 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).
> 
> This adds support for it to KVM, exactly the same way as the existing 
> support for SYSTEM_RESET2 as added in commits d43583b890e7 ("KVM: arm64: 
> Expose PSCI SYSTEM_RESET2 call to the guest") and 34739fd95fab ("KVM: 
> arm64: Indicate SYSTEM_RESET2 in kvm_run::system_event flags field").
> 
> Back then, KVM was unconditionally bumped to expose PSCI v1.1. This 
> means that a kernel upgrade causes guest visible behaviour changes 
> without any explicit opt-in from the VMM, which is... unconventional. In 
> some cases, a PSCI update isn't just about new optional calls; PSCI v1.2 
> for example adds a new permitted error return from the existing CPU_ON 
> function.
> 
> There *is* a way for a VMM to opt *out* of newer PSCI versions... by 
> setting a per-vCPU "special" register that actually ends up setting the 
> PSCI version KVM-wide. Quite why this isn't just a simple KVM_CAP, I 
> have no idea.

Because the expectations are that the VMM can blindly save/restore the
guest's state, including the PSCI version, and restore that blindly.
KVM CAPs are just a really bad design pattern for this sort of things.

	M.

-- 
Without deviation from the norm, progress is not possible.