[kvmtool PATCH v9 06/15] PCI: Only allocate IRQ routing entry when available

On Wed, Mar 29 2017 at  9:44:47 pm BST, Andr? Przywara [off-list ref] wrote:

Hi Marc,

thanks for having a look!

On 29/03/17 09:30, Marc Zyngier wrote:

quoted

On 02/02/17 16:32, Andre Przywara wrote:

quoted

If we need to inject an MSI into the guest, we rely at the moment on a
working GSI MSI routing functionality. However we can get away without
IRQ routing, if the host supports MSI injection via the KVM_SIGNAL_MSI
ioctl.
So we try the GSI routing first, but if that fails due to a missing
IRQ routing functionality, we fall back to KVM_SIGNAL_MSI (if that is
supported).

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
---
 virtio/pci.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/virtio/pci.c b/virtio/pci.c
index 7cc0ba4..98bf6b7 100644
--- a/virtio/pci.c
+++ b/virtio/pci.c

@@ -193,8 +193,14 @@ static bool virtio_pci__specific_io_out(struct kvm *kvm, struct virtio_device *v
 
 			gsi = irq__add_msix_route(kvm,
 						  &vpci->msix_table[vec].msg);
-			if (gsi >= 0)
+			if (gsi >= 0) {
 				vpci->config_gsi = gsi;
+				break;
+			}
+			if (gsi == -ENXIO &&
+			    vpci->features & VIRTIO_PCI_F_SIGNAL_MSI)
+					break;
+			die("failed to configure MSIs");
 			break;
 		case VIRTIO_MSI_QUEUE_VECTOR:
 			vec = ioport__read16(data);

@@ -205,8 +211,14 @@ static bool virtio_pci__specific_io_out(struct kvm *kvm, struct virtio_device *v
 
 			gsi = irq__add_msix_route(kvm,
 						  &vpci->msix_table[vec].msg);
-			if (gsi < 0)
+			if (gsi < 0) {
+				if (gsi == -ENXIO &&
+				    vpci->features & VIRTIO_PCI_F_SIGNAL_MSI)
+					break;
+
+				die("failed to configure MSIs");
 				break;
+			}
 			vpci->gsis[vpci->queue_selector] = gsi;

Is it really expected to find -ENXIO in this array instead of a GSI?

Not sure I get this "in this array" comment here. Any negative return
value ends up in a break, without writing it anywhere.
Later on we check for VIRTIO_PCI_F_SIGNAL_MSI first before referencing
the array. This is admittedly a bit convoluted.
So shall I add comments here? Or rewrite the if statement to make this
more obvious?

Ah! Of course you're right, I completely misread the code. Maybe indeed
rewriting a bit to make it more obvious:

	if (gsi == -ENXIO && vpci->features & VIRTIO_PCI_F_SIGNAL_MSI)
		break;
        if (gsi < 0) {
        	die("failed to configure MSIs");
                break
        }

which is closer to the first.

Thanks,

	M.
-- 
Jazz is not dead, it just smell funny.