irqchip/gic-v3-its: Align PCI Multi-MSI allocation on their size

The way we allocate events works fine in most cases, except
when multiple PCI devices share an ITS-visible DevID, and that
one of them is trying to use MultiMSI allocation.

In that case, our allocation is not guaranteed to be zero-based
anymore, and we have to make sure we allocate it on a boundary
that is compatible with the PCI Multi-MSI constraints.

Fix this by allocating the full region upfront instead of iterating
over the number of MSIs. MSI-X are always allocated one by one,
so this shouldn't change anything on that front.

Fixes: b48ac83d ("irqchip: GICv3: ITS: MSI support")
Cc: stable@vger.kernel.org
Reported-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Tested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

drivers/irqchip/irq-gic-v3-its.c

@@ -2399,13 +2399,14 @@ static void its_free_device(struct its_device *its_dev)
 	kfree(its_dev);
 }
 
-static int its_alloc_device_irq(struct its_device *dev, irq_hw_number_t *hwirq)
+static int its_alloc_device_irq(struct its_device *dev, int nvecs, irq_hw_number_t *hwirq)
 {
 	int idx;
 
-	idx = find_first_zero_bit(dev->event_map.lpi_map,
-				  dev->event_map.nr_lpis);
-	if (idx == dev->event_map.nr_lpis)
+	idx = bitmap_find_free_region(dev->event_map.lpi_map,
+				      dev->event_map.nr_lpis,
+				      get_count_order(nvecs));
+	if (idx < 0)
 		return -ENOSPC;
 
 	*hwirq = dev->event_map.lpi_base + idx;
@@ -2501,21 +2502,21 @@ static int its_irq_domain_alloc(struct irq_domain *domain, unsigned int virq,
 	int err;
 	int i;
 
-	for (i = 0; i < nr_irqs; i++) {
-		err = its_alloc_device_irq(its_dev, &hwirq);
-		if (err)
-			return err;
+	err = its_alloc_device_irq(its_dev, nr_irqs, &hwirq);
+	if (err)
+		return err;
 
-		err = its_irq_gic_domain_alloc(domain, virq + i, hwirq);
+	for (i = 0; i < nr_irqs; i++) {
+		err = its_irq_gic_domain_alloc(domain, virq + i, hwirq + i);
 		if (err)
 			return err;
 
 		irq_domain_set_hwirq_and_chip(domain, virq + i,
-					      hwirq, &its_irq_chip, its_dev);
+					      hwirq + i, &its_irq_chip, its_dev);
 		irqd_set_single_target(irq_desc_get_irq_data(irq_to_desc(virq + i)));
 		pr_debug("ID:%d pID:%d vID:%d\n",
-			 (int)(hwirq - its_dev->event_map.lpi_base),
-			 (int) hwirq, virq + i);
+			 (int)(hwirq + i - its_dev->event_map.lpi_base),
+			 (int)(hwirq + i), virq + i);
 	}
 
 	return 0;