arm/arm64: KVM: Handle out-of-RAM cache maintenance as a NOP

So far, our handling of cache maintenance by VA has been pretty
simple: Either the access is in the guest RAM and generates a S2
fault, which results in the page being mapped RW, or we go down
the io_mem_abort() path, and nuke the guest.

The first one is fine, but the second one is extremely weird.
Treating the CM as an I/O is wrong, and nothing in the ARM ARM
indicates that we should generate a fault for something that
cannot end-up in the cache anyway (even if the guest maps it,
it will keep on faulting at stage-2 for emulation).

So let's just skip this instruction, and let the guest get away
with it.

Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

arch/arm/include/asm/kvm_emulate.h

@@ -138,6 +138,11 @@ static inline bool kvm_vcpu_dabt_iss1tw(struct kvm_vcpu *vcpu)
 	return kvm_vcpu_get_hsr(vcpu) & HSR_DABT_S1PTW;
 }
 
+static inline bool kvm_vcpu_dabt_is_cm(struct kvm_vcpu *vcpu)
+{
+	return !!(kvm_vcpu_get_hsr(vcpu) & HSR_DABT_CM);
+}
+
 /* Get Access Size from a data abort */
 static inline int kvm_vcpu_dabt_get_as(struct kvm_vcpu *vcpu)
 {

arch/arm/kvm/mmu.c

@@ -1430,6 +1430,22 @@ int kvm_handle_guest_abort(struct kvm_vcpu *vcpu, struct kvm_run *run)
 			goto out_unlock;
 		}
 
+		/*
+		 * Check for a cache maintenance operation. Since we
+		 * ended-up here, we know it is outside of any memory
+		 * slot. But we can't find out if that is for a device,
+		 * or if the guest is just being stupid. The only thing
+		 * we know for sure is that this range cannot be cached.
+		 *
+		 * So let's assume that the guest is just being
+		 * cautious, and skip the instruction.
+		 */
+		if (kvm_vcpu_dabt_is_cm(vcpu)) {
+			kvm_skip_instr(vcpu, kvm_vcpu_trap_il_is32bit(vcpu));
+			ret = 1;
+			goto out_unlock;
+		}
+
 		/*
 		 * The IPA is reported as [MAX:12], so we need to
 		 * complement it with the bottom 12 bits from the

arch/arm64/include/asm/kvm_emulate.h

@@ -189,6 +189,11 @@ static inline bool kvm_vcpu_dabt_iss1tw(const struct kvm_vcpu *vcpu)
 	return !!(kvm_vcpu_get_hsr(vcpu) & ESR_ELx_S1PTW);
 }
 
+static inline bool kvm_vcpu_dabt_is_cm(const struct kvm_vcpu *vcpu)
+{
+	return !!(kvm_vcpu_get_hsr(vcpu) & ESR_ELx_CM);
+}
+
 static inline int kvm_vcpu_dabt_get_as(const struct kvm_vcpu *vcpu)
 {
 	return 1 << ((kvm_vcpu_get_hsr(vcpu) & ESR_ELx_SAS) >> ESR_ELx_SAS_SHIFT);