KVM: arm64: Work around C1-Pro erratum 4193714 for protected guests

C1-Pro cores with SME have an erratum where TLBI+DSB does not complete
all outstanding SME accesses. Instead a DSB needs to be executed on the
affected CPUs. The implication is that pages cannot be unmapped from the
host Stage 2 and then provided to a protected guest or to the
hypervisor. Host SME accesses may still complete after this point.

This erratum breaks pKVM's guarantees, and the workaround is hard to
implement as EL2 and EL1 share a security state meaning EL1 can mask
IPIs sent by EL2, leading to interrupt blackouts.

Instead, do this in EL3. This has the advantage of a separate security
state, meaning lower EL cannot mask the IPI. It is also simpler for EL3
to know about CPUs that are off or in PSCI's CPU_SUSPEND.

Add the needed hook to host_stage2_set_owner_metadata_locked(). This
covers the cases where the host loses access to a page:

  __pkvm_host_donate_guest()
  __pkvm_guest_unshare_host()
  host_stage2_set_owner_locked() when owner_id == PKVM_ID_HYP

Since pKVM relies on the firmware call for correctness, check for the
firmware counterpart during protected KVM initialisation and fail the
pKVM initialisation if it is missing.

Signed-off-by: James Morse <james.morse@arm.com>
Co-developed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Oliver Upton <oupton@kernel.org>
Cc: Will Deacon <will@kernel.org>
Cc: Vincent Donnefort <vdonnefort@google.com>
Cc: Lorenzo Pieralisi <lpieralisi@kernel.org>
Cc: Sudeep Holla <sudeep.holla@kernel.org>
Link: https://patch.msgid.link/20260505165205.2690919-1-catalin.marinas@arm.com
Signed-off-by: Marc Zyngier <maz@kernel.org>

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