lguest: fix for CONFIG_SPARSE_IRQ=y

Impact: remove lots of lguest boot WARN_ON() when CONFIG_SPARSE_IRQ=y

We now need to call irq_to_desc_alloc_cpu() before
set_irq_chip_and_handler_name(), but we can't do that from init_IRQ (no
kmalloc available).

So do it as we use interrupts instead.  Also means we only alloc for
irqs we use, which was the intent of CONFIG_SPARSE_IRQ anyway.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Ingo Molnar <mingo@redhat.com>

arch/x86/lguest/boot.c

@@ -594,19 +594,21 @@ static void __init lguest_init_IRQ(void)
 		/* Some systems map "vectors" to interrupts weirdly.  Lguest has
 		 * a straightforward 1 to 1 mapping, so force that here. */
 		__get_cpu_var(vector_irq)[vector] = i;
-		if (vector != SYSCALL_VECTOR) {
-			set_intr_gate(vector,
-				      interrupt[vector-FIRST_EXTERNAL_VECTOR]);
-			set_irq_chip_and_handler_name(i, &lguest_irq_controller,
-						      handle_level_irq,
-						      "level");
-		}
+		if (vector != SYSCALL_VECTOR)
+			set_intr_gate(vector, interrupt[i]);
 	}
 	/* This call is required to set up for 4k stacks, where we have
 	 * separate stacks for hard and soft interrupts. */
 	irq_ctx_init(smp_processor_id());
 }
 
+void lguest_setup_irq(unsigned int irq)
+{
+	irq_to_desc_alloc_cpu(irq, 0);
+	set_irq_chip_and_handler_name(irq, &lguest_irq_controller,
+				      handle_level_irq, "level");
+}
+
 /*
  * Time.
  *

drivers/lguest/lguest_device.c

@@ -212,6 +212,9 @@ static void lg_notify(struct virtqueue *vq)
 	hcall(LHCALL_NOTIFY, lvq->config.pfn << PAGE_SHIFT, 0, 0);
 }
 
+/* An extern declaration inside a C file is bad form.  Don't do it. */
+extern void lguest_setup_irq(unsigned int irq);
+
 /* This routine finds the first virtqueue described in the configuration of
  * this device and sets it up.
  *
@@ -266,6 +269,9 @@ static struct virtqueue *lg_find_vq(struct virtio_device *vdev,
 		goto unmap;
 	}
 
+	/* Make sure the interrupt is allocated. */
+	lguest_setup_irq(lvq->config.irq);
+
 	/* Tell the interrupt for this virtqueue to go to the virtio_ring
 	 * interrupt handler. */
 	/* FIXME: We used to have a flag for the Host to tell us we could use