[PATCH v1 4/4] vfio/pci: enable MSI-X in interrupt restoring on dynamic allocation

[Jing Liu <jing2.liu@intel.com>]

During migration restoring, vfio_enable_vectors() is called to restore
enabling MSI-X interrupts for assigned devices. It sets the range from
0 to nr_vectors to kernel to enable MSI-X and the vectors unmasked in
guest. During the MSI-X enabling, all the vectors within the range are
allocated according to the VFIO_DEVICE_SET_IRQS ioctl.

When dynamic MSI-X allocation is supported, we only want the guest
unmasked vectors being allocated and enabled. Use vector 0 with an
invalid fd to get MSI-X enabled, after that, all the vectors can be
allocated in need.

Signed-off-by: Jing Liu <jing2.liu@intel.com>
---
Changes since RFC v1:
- Revise the comments. (Alex)
- Call the new helper function in previous patch to enable MSI-X. (Alex)
---
 hw/vfio/pci.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
index e24c21241a0c..3c37d036e727 100644
--- a/hw/vfio/pci.c
+++ b/hw/vfio/pci.c
@@ -408,6 +408,23 @@ static int vfio_enable_vectors(VFIOPCIDevice *vdev, bool msix)
     int ret = 0, i, argsz;
     int32_t *fds;
 
+    /*
+     * If dynamic MSI-X allocation is supported, the vectors to be allocated
+     * and enabled can be scattered. Before kernel enabling MSI-X, setting
+     * nr_vectors causes all these vectors to be allocated on host.
+     *
+     * To keep allocation as needed, use vector 0 with an invalid fd to get
+     * MSI-X enabled first, then set vectors with a potentially sparse set of
+     * eventfds to enable interrupts only when enabled in guest.
+     */
+    if (msix && !vdev->msix->noresize) {
+        ret = vfio_enable_msix_no_vec(vdev);
+
+        if (ret) {
+            return ret;
+        }
+    }
+
     argsz = sizeof(*irq_set) + (vdev->nr_vectors * sizeof(*fds));
 
     irq_set = g_malloc0(argsz);
-- 
2.27.0