KVM: arm64: Start trapping ID registers for 32 bit guests

To date KVM has not trapped ID register accesses from AArch32, meaning
that guests get an unconstrained view of what hardware supports. This
can be a serious problem because we try to base the guest's feature
registers on values that are safe system-wide. Furthermore, KVM does not
implement the latest ISA in the PMU and Debug architecture, so we
constrain these fields to supported values.

Since KVM now correctly handles CP15 and CP10 register traps, we no
longer need to clear HCR_EL2.TID3 for 32 bit guests and will instead
emulate reads with their safe values.
Signed-off-by: Oliver Upton <oupton@google.com>
Reviewed-by: Reiji Watanabe <reijiw@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220503060205.2823727-6-oupton@google.com

arch/arm64/include/asm/kvm_arm.h

@@ -80,11 +80,12 @@
  * FMO:		Override CPSR.F and enable signaling with VF
  * SWIO:	Turn set/way invalidates into set/way clean+invalidate
  * PTW:		Take a stage2 fault if a stage1 walk steps in device memory
+ * TID3:	Trap EL1 reads of group 3 ID registers
  */
 #define HCR_GUEST_FLAGS (HCR_TSC | HCR_TSW | HCR_TWE | HCR_TWI | HCR_VM | \
 			 HCR_BSU_IS | HCR_FB | HCR_TACR | \
 			 HCR_AMO | HCR_SWIO | HCR_TIDCP | HCR_RW | HCR_TLOR | \
-			 HCR_FMO | HCR_IMO | HCR_PTW )
+			 HCR_FMO | HCR_IMO | HCR_PTW | HCR_TID3 )
 #define HCR_VIRT_EXCP_MASK (HCR_VSE | HCR_VI | HCR_VF)
 #define HCR_HOST_NVHE_FLAGS (HCR_RW | HCR_API | HCR_APK | HCR_ATA)
 #define HCR_HOST_NVHE_PROTECTED_FLAGS (HCR_HOST_NVHE_FLAGS | HCR_TSC)

arch/arm64/include/asm/kvm_emulate.h

@@ -86,13 +86,6 @@ static inline void vcpu_reset_hcr(struct kvm_vcpu *vcpu)
 
 	if (vcpu_el1_is_32bit(vcpu))
 		vcpu->arch.hcr_el2 &= ~HCR_RW;
-	else
-		/*
-		 * TID3: trap feature register accesses that we virtualise.
-		 * For now this is conditional, since no AArch32 feature regs
-		 * are currently virtualised.
-		 */
-		vcpu->arch.hcr_el2 |= HCR_TID3;
 
 	if (cpus_have_const_cap(ARM64_MISMATCHED_CACHE_TYPE) ||
 	    vcpu_el1_is_32bit(vcpu))