[PULL 2/2] vfio/pci: Guard accel_irqchip_begin_route_changes() calls

["Cédric Le Goater" <clg@redhat.com>]

Since commit 49b2dcbd2422 ("accel/accel-irq: add generic
begin_route_changes"), accel_irqchip_begin_route_changes() aborts when
no accelerator irqchip is available. This causes a fatal error when
running VFIO passthrough devices under TCG emulation:

  qemu-system-aarch64: can't initiate route change, no accel irqchip available

The previous kvm_irqchip_begin_route_changes() was a simple inline
that did not have a fatal path. The VFIO code already handles the
absence of KVM MSI routing gracefully by falling back to userspace
handling, but the new generic function aborts before that fallback
can take effect.

Guard the call sites in hw/vfio/pci.c with
accel_msi_via_irqfd_enabled() so that route changes are only
initiated when an accelerator irqchip is actually present.

Fixes: 49b2dcbd2422 ("accel/accel-irq: add generic begin_route_changes")
Cc: Magnus Kulke <magnuskulke@linux.microsoft.com>
Link: https://lore.kernel.org/qemu-devel/20260721105026.3932297-1-clg@redhat.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>
---
 hw/vfio/pci.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
index b5280c3d2a366cd7cff4069936858ac651afeb6d..428ab2f06983ba9ecb306975a4aca7f6efe1b855 100644
--- a/hw/vfio/pci.c
+++ b/hw/vfio/pci.c
@@ -699,7 +699,7 @@ static int vfio_msix_vector_do_use(PCIDevice *pdev, unsigned int nr,
         if (msg) {
             if (vdev->defer_kvm_irq_routing) {
                 vfio_pci_add_kvm_msi_virq(vdev, vector, nr, true);
-            } else {
+            } else if (accel_msi_via_irqfd_enabled()) {
                 vfio_route_change = accel_irqchip_begin_route_changes();
                 vfio_pci_add_kvm_msi_virq(vdev, vector, nr, true);
                 accel_irqchip_commit_route_changes(&vfio_route_change);
@@ -801,7 +801,9 @@ void vfio_pci_prepare_kvm_msi_virq_batch(VFIOPCIDevice *vdev)
 {
     assert(!vdev->defer_kvm_irq_routing);
     vdev->defer_kvm_irq_routing = true;
-    vfio_route_change = accel_irqchip_begin_route_changes();
+    if (accel_msi_via_irqfd_enabled()) {
+        vfio_route_change = accel_irqchip_begin_route_changes();
+    }
 }
 
 void vfio_pci_commit_kvm_msi_virq_batch(VFIOPCIDevice *vdev)
-- 
2.55.0