On Mon, 18 Mar 2024 16:14:22 +0000, David Woodhouse <dwmw2@xxxxxxxxxxxxx> 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.
