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From: David Woodhouse <dwmw2@infradead.org>
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Subject: [RFC PATCH v2 0/4] arm64: Add PSCI v1.3 SYSTEM_OFF2 support for hibernation
Date: Mon, 18 Mar 2024 16:14:22 +0000
Message-ID: <20240318164646.1010092-1-dwmw2@infradead.org>
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Sender: David Woodhouse <dwmw2@infradead.org>

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. There *is* a KVM_CAP_ARM_PSCI_0_2 but that's just for 0.1 
vs. 0.2+, not the specific v0.2+ version that's exposed.

Since the SYSTEM_OFF2 call is optional and discoverable through the 
PSCI_FEATURES call, I'm electing not to touch the PSCI versioning 
awfulness at all. Like the existing SYSTEM_RESET2, there's a KVM_CAP to 
enable it explicitly (as it's an optional call even in v1.3), and like 
the existing SYSTEM_RESET2 it doesn't depend on the advertised PSCI 
version.

For the guest side, add a new SYS_OFF_MODE_POWER_OFF handler with higher 
priority than the EFI one, but which *only* triggers when there's a 
hibernation in progress. There are other ways to do this (see the commit
message for more details) but this seemed like the simplest.

Version 2 of the patch series splits out the psci.h definitions into a 
separate commit (a dependency for both the guest and KVM side), and adds 
definitions for the other new functions added in v1.3. It also moves the 
pKVM psci-relay support to a separate commit; although in arch/arm64/kvm 
that's actually about the *guest* side of SYSTEM_OFF2 (i.e. using it
from the host kernel, relayed through nVHE).

David Woodhouse (4):
      firmware/psci: Add definitions for PSCI v1.3 specification (ALPHA)
      KVM: arm64: Add PSCI SYSTEM_OFF2 function for hibernation
      KVM: arm64: nvhe: Pass through PSCI v1.3 SYSTEM_OFF2 call
      arm64: Use SYSTEM_OFF2 PSCI call to power off for hibernate

 Documentation/virt/kvm/api.rst       | 11 +++++++++++
 arch/arm64/include/asm/kvm_host.h    |  2 ++
 arch/arm64/include/uapi/asm/kvm.h    |  6 ++++++
 arch/arm64/kvm/arm.c                 |  5 +++++
 arch/arm64/kvm/hyp/nvhe/psci-relay.c |  2 ++
 arch/arm64/kvm/psci.c                | 37 ++++++++++++++++++++++++++++++++++++
 drivers/firmware/psci/psci.c         | 35 ++++++++++++++++++++++++++++++++++
 include/uapi/linux/kvm.h             |  1 +
 include/uapi/linux/psci.h            | 20 +++++++++++++++++++
 kernel/power/hibernate.c             |  5 ++++-
 10 files changed, 123 insertions(+), 1 deletion(-)