In order to detect whether a GICv3 CPU interface is MMIO capable,
we switch ICC_SRE_EL1.SRE to 0 and check whether it sticks.

However, this is only possible if *ALL* of the HCR_EL2 interrupt
overrides are set, and the CPU is perfectly allowed to ignore
the write to ICC_SRE_EL1 otherwise. This leads KVM to pretend
that a whole bunch of ARMv8.0 CPUs aren't MMIO-capable, and
breaks VMs that should work correctly otherwise.

Fix this by setting IMO/FMO/IMO before touching ICC_SRE_EL1,
and clear them afterwards. This allows us to reliably detect
the CPU interface capabilities.
Tested-by: Shameerali Kolothum Thodi <shameerali.kolothum.thodi@huawei.com>
Fixes: 9739f6ef ("KVM: arm64: Workaround firmware wrongly advertising GICv2-on-v3 compatibility")
Signed-off-by: Marc Zyngier <maz@kernel.org>