[RFC PATCH v2 3/3] vfio: Avoid disabling and enabling vectors repeatedly in VFIO migration

[Shenming Lu <lushenming@huawei.com>]

Different from the normal situation when the guest starts, we can
know the max unmasked vetctor (at the beginning) after msix_load()
in VFIO migration. So in order to avoid ineffectively disabling and
enabling vectors repeatedly, let's allocate all needed vectors first
and then enable these unmasked vectors one by one without disabling.

Signed-off-by: Shenming Lu <lushenming@huawei.com>
---
 hw/pci/msix.c         | 17 +++++++++++++++++
 hw/vfio/pci.c         | 10 ++++++++--
 include/hw/pci/msix.h |  2 ++
 3 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/hw/pci/msix.c b/hw/pci/msix.c
index 67e34f34d6..bf291d3ff8 100644
--- a/hw/pci/msix.c
+++ b/hw/pci/msix.c
@@ -557,6 +557,23 @@ unsigned int msix_nr_vectors_allocated(const PCIDevice *dev)
     return dev->msix_entries_nr;
 }
 
+int msix_get_max_unmasked_vector(PCIDevice *dev)
+{
+    int max_unmasked_vector = -1;
+    int vector;
+
+    if ((dev->config[dev->msix_cap + MSIX_CONTROL_OFFSET] &
+        (MSIX_ENABLE_MASK | MSIX_MASKALL_MASK)) == MSIX_ENABLE_MASK) {
+        for (vector = 0; vector < dev->msix_entries_nr; vector++) {
+            if (!msix_is_masked(dev, vector)) {
+                max_unmasked_vector = vector;
+            }
+        }
+    }
+
+    return max_unmasked_vector;
+}
+
 static int msix_set_notifier_for_vector(PCIDevice *dev, unsigned int vector)
 {
     MSIMessage msg;
diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
index 51dc373695..e755ed2514 100644
--- a/hw/vfio/pci.c
+++ b/hw/vfio/pci.c
@@ -568,6 +568,9 @@ static void vfio_msix_vector_release(PCIDevice *pdev, unsigned int nr)
 
 static void vfio_msix_enable(VFIOPCIDevice *vdev)
 {
+    int max_unmasked_vector = msix_get_max_unmasked_vector(&vdev->pdev);
+    unsigned int used_vector = MAX(max_unmasked_vector, 0);
+
     vfio_disable_interrupts(vdev);
 
     vdev->msi_vectors = g_new0(VFIOMSIVector, vdev->msix->entries);
@@ -586,9 +589,12 @@ static void vfio_msix_enable(VFIOPCIDevice *vdev)
      * triggering to userspace, then immediately release the vector, leaving
      * the physical device with no vectors enabled, but MSI-X enabled, just
      * like the guest view.
+     * If there are unmasked vectors (such as in migration) which will be
+     * enabled soon, we can allocate them here to avoid ineffectively disabling
+     * and enabling vectors repeatedly later.
      */
-    vfio_msix_vector_do_use(&vdev->pdev, 0, NULL, NULL);
-    vfio_msix_vector_release(&vdev->pdev, 0);
+    vfio_msix_vector_do_use(&vdev->pdev, used_vector, NULL, NULL);
+    vfio_msix_vector_release(&vdev->pdev, used_vector);
 
     if (msix_set_vector_notifiers(&vdev->pdev, vfio_msix_vector_use,
                                   vfio_msix_vector_release, NULL)) {
diff --git a/include/hw/pci/msix.h b/include/hw/pci/msix.h
index 4c4a60c739..4bfb463fa6 100644
--- a/include/hw/pci/msix.h
+++ b/include/hw/pci/msix.h
@@ -23,6 +23,8 @@ void msix_uninit_exclusive_bar(PCIDevice *dev);
 
 unsigned int msix_nr_vectors_allocated(const PCIDevice *dev);
 
+int msix_get_max_unmasked_vector(PCIDevice *dev);
+
 void msix_save(PCIDevice *dev, QEMUFile *f);
 void msix_load(PCIDevice *dev, QEMUFile *f);
 
-- 
2.19.1