KVM: arm64: Make build-time check of RES0/RES1 bits optional

In order to ease the transition towards a state of absolute
paranoia where all RES0/RES1 bits gets checked against what
KVM know of them, make the checks optional and guarded by a
config symbol (CONFIG_KVM_ARM64_RES_BITS_PARANOIA) default to n.
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/kvm/87frxka7ud.wl-maz@kernel.org/Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

arch/arm64/kvm/Kconfig

@@ -67,4 +67,15 @@ config PROTECTED_NVHE_STACKTRACE
 
 	  If unsure, or not using protected nVHE (pKVM), say N.
 
+config KVM_ARM64_RES_BITS_PARANOIA
+	bool "Build-time check of RES0/RES1 bits"
+	depends on KVM
+	default n
+	help
+	  Say Y here to validate that KVM's knowledge of most system
+	  registers' RES0/RES1 bits matches when the rest of the kernel
+	  defines. Expect the build to fail badly if you enable this.
+
+	  Just say N.
+
 endif # VIRTUALIZATION

arch/arm64/kvm/check-res-bits.h

@@ -21,6 +21,8 @@
  */
 static inline void check_res_bits(void)
 {
+#ifdef CONFIG_KVM_ARM64_RES_BITS_PARANOIA
+
 	BUILD_BUG_ON(OSDTRRX_EL1_RES0		!= (GENMASK_ULL(63, 32)));
 	BUILD_BUG_ON(MDCCINT_EL1_RES0		!= (GENMASK_ULL(63, 31) | GENMASK_ULL(28, 0)));
 	BUILD_BUG_ON(MDSCR_EL1_RES0		!= (GENMASK_ULL(63, 36) | GENMASK_ULL(28, 28) | GENMASK_ULL(25, 24) | GENMASK_ULL(20, 20) | GENMASK_ULL(18, 16) | GENMASK_ULL(11, 7) | GENMASK_ULL(5, 1)));
@@ -118,4 +120,6 @@ static inline void check_res_bits(void)
 	BUILD_BUG_ON(TRBMAR_EL1_RES0		!= (GENMASK_ULL(63, 12)));
 	BUILD_BUG_ON(TRBTRG_EL1_RES0		!= (GENMASK_ULL(63, 32)));
 	BUILD_BUG_ON(TRBIDR_EL1_RES0		!= (GENMASK_ULL(63, 12) | GENMASK_ULL(7, 6)));
+
+#endif
 }