powerpc/xive: Introduce XIVE_IPI_HW_IRQ

The XIVE driver deals with CPU IPIs in a peculiar way. Each CPU has
its own XIVE IPI interrupt allocated at the HW level, for PowerNV, or
at the hypervisor level for pSeries. In practice, these interrupts are
not always used. pSeries/PowerVM prefers local doorbells for local
threads since they are faster. On PowerNV, global doorbells are also
preferred for the same reason.

The mapping in the Linux is reduced to a single interrupt using HW
interrupt number 0 and a custom irq_chip to handle EOI. This can cause
performance issues in some benchmark (ipistorm) on multichip systems.

Clarify the use of the 0 value, it will help in improving multichip
support.
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201210171450.1933725-4-clg@kaod.org

arch/powerpc/sysdev/xive/common.c

@@ -1142,7 +1142,7 @@ static void __init xive_request_ipi(void)
 		return;
 
 	/* Initialize it */
-	virq = irq_create_mapping(xive_irq_domain, 0);
+	virq = irq_create_mapping(xive_irq_domain, XIVE_IPI_HW_IRQ);
 	xive_ipi_irq = virq;
 
 	WARN_ON(request_irq(virq, xive_muxed_ipi_action,
@@ -1242,7 +1242,7 @@ static int xive_irq_domain_map(struct irq_domain *h, unsigned int virq,
 
 #ifdef CONFIG_SMP
 	/* IPIs are special and come up with HW number 0 */
-	if (hw == 0) {
+	if (hw == XIVE_IPI_HW_IRQ) {
 		/*
 		 * IPIs are marked per-cpu. We use separate HW interrupts under
 		 * the hood but associated with the same "linux" interrupt
@@ -1271,7 +1271,7 @@ static void xive_irq_domain_unmap(struct irq_domain *d, unsigned int virq)
 	if (!data)
 		return;
 	hw_irq = (unsigned int)irqd_to_hwirq(data);
-	if (hw_irq)
+	if (hw_irq != XIVE_IPI_HW_IRQ)
 		xive_irq_free_data(virq);
 }
 
@@ -1421,7 +1421,7 @@ static void xive_flush_cpu_queue(unsigned int cpu, struct xive_cpu *xc)
 		 * Ignore anything that isn't a XIVE irq and ignore
 		 * IPIs, so can just be dropped.
 		 */
-		if (d->domain != xive_irq_domain || hw_irq == 0)
+		if (d->domain != xive_irq_domain || hw_irq == XIVE_IPI_HW_IRQ)
 			continue;
 
 		/*
@@ -1655,7 +1655,7 @@ static int xive_core_debug_show(struct seq_file *m, void *private)
 		hw_irq = (unsigned int)irqd_to_hwirq(d);
 
 		/* IPIs are special (HW number 0) */
-		if (hw_irq)
+		if (hw_irq != XIVE_IPI_HW_IRQ)
 			xive_debug_show_irq(m, hw_irq, d);
 	}
 	return 0;

arch/powerpc/sysdev/xive/xive-internal.h

@@ -5,6 +5,8 @@
 #ifndef __XIVE_INTERNAL_H
 #define __XIVE_INTERNAL_H
 
+#define XIVE_IPI_HW_IRQ		0 /* interrupt source # for IPIs */
+
 /*
  * A "disabled" interrupt should never fire, to catch problems
  * we set its logical number to this