KVM: PPC: Book3S HV P9: Restrict DSISR canary workaround to processors that require it

Use CPU_FTR_P9_RADIX_PREFETCH_BUG to apply the workaround, to test for
DD2.1 and below processors. This saves a mtSPR in guest entry.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211123095231.1036501-35-npiggin@gmail.com

arch/powerpc/kvm/book3s_hv.c

@@ -1590,7 +1590,8 @@ static int kvmppc_handle_exit_hv(struct kvm_vcpu *vcpu,
 		unsigned long vsid;
 		long err;
 
-		if (vcpu->arch.fault_dsisr == HDSISR_CANARY) {
+		if (cpu_has_feature(CPU_FTR_P9_RADIX_PREFETCH_BUG) &&
+		    unlikely(vcpu->arch.fault_dsisr == HDSISR_CANARY)) {
 			r = RESUME_GUEST; /* Just retry if it's the canary */
 			break;
 		}

arch/powerpc/kvm/book3s_hv_p9_entry.c

@@ -683,9 +683,11 @@ int kvmhv_vcpu_entry_p9(struct kvm_vcpu *vcpu, u64 time_limit, unsigned long lpc
 	 * HDSI which should correctly update the HDSISR the second time HDSI
 	 * entry.
 	 *
-	 * Just do this on all p9 processors for now.
+	 * The "radix prefetch bug" test can be used to test for this bug, as
+	 * it also exists fo DD2.1 and below.
 	 */
-	mtspr(SPRN_HDSISR, HDSISR_CANARY);
+	if (cpu_has_feature(CPU_FTR_P9_RADIX_PREFETCH_BUG))
+		mtspr(SPRN_HDSISR, HDSISR_CANARY);
 
 	mtspr(SPRN_SPRG0, vcpu->arch.shregs.sprg0);
 	mtspr(SPRN_SPRG1, vcpu->arch.shregs.sprg1);