KVM: PPC: Book3S HV: Workaround POWER9 DD1.0 bug causing IPB bit loss

This adds a workaround for a bug in POWER9 DD1 chips where changing
the CPPR (Current Processor Priority Register) can cause bits in the
IPB (Interrupt Pending Buffer) to get lost.  Thankfully it only
happens when manually manipulating CPPR which is quite rare.  When it
does happen it can cause interrupts to be delayed or lost.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

arch/powerpc/kvm/book3s_xive_template.c

@@ -16,7 +16,16 @@ static void GLUE(X_PFX,ack_pending)(struct kvmppc_xive_vcpu *xc)
 	u8 cppr;
 	u16 ack;
 
-	/* XXX DD1 bug workaround: Check PIPR vs. CPPR first ! */
+	/*
+	 * DD1 bug workaround: If PIPR is less favored than CPPR
+	 * ignore the interrupt or we might incorrectly lose an IPB
+	 * bit.
+	 */
+	if (cpu_has_feature(CPU_FTR_POWER9_DD1)) {
+		u8 pipr = __x_readb(__x_tima + TM_QW1_OS + TM_PIPR);
+		if (pipr >= xc->hw_cppr)
+			return;
+	}
 
 	/* Perform the acknowledge OS to register cycle. */
 	ack = be16_to_cpu(__x_readw(__x_tima + TM_SPC_ACK_OS_REG));