Re: [RFC PATCH v2 0/4] arm64: Add PSCI v1.3 SYSTEM_OFF2 support for hibernation

Marc Zyngier
2024-03-18T16:57:06Z

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.