arm64: Allow arm64_sw.hvhe on command line

Add the arm64_sw.hvhe=1 option to force the use of the hVHE mode
in the hypervisor code only.

This enables the hVHE mode of operation when using KVM on VHE
hardware.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20230609162200.2024064-17-maz@kernel.org
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

arch/arm64/kernel/idreg-override.c

@@ -138,11 +138,22 @@ static const struct ftr_set_desc smfr0 __initconst = {
 	},
 };
 
+static bool __init hvhe_filter(u64 val)
+{
+	u64 mmfr1 = read_sysreg(id_aa64mmfr1_el1);
+
+	return (val == 1 &&
+		lower_32_bits(__boot_status) == BOOT_CPU_MODE_EL2 &&
+		cpuid_feature_extract_unsigned_field(mmfr1,
+						     ID_AA64MMFR1_EL1_VH_SHIFT));
+}
+
 static const struct ftr_set_desc sw_features __initconst = {
 	.name		= "arm64_sw",
 	.override	= &arm64_sw_feature_override,
 	.fields		= {
 		FIELD("nokaslr", ARM64_SW_FEATURE_OVERRIDE_NOKASLR, NULL),
+		FIELD("hvhe", ARM64_SW_FEATURE_OVERRIDE_HVHE, hvhe_filter),
 		{}
 	},
 };