[PATCH RFC v1 2/3] vfio/pci: enable vector on dynamic MSI-X allocation

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

The vector_use callback is used to enable vector that is unmasked in
guest. The kernel used to only support static MSI-X allocation. When
allocating a new interrupt using "static MSI-X allocation" kernels,
Qemu first disables all previously allocated vectors and then
re-allocates all including the new one. The nr_vectors of VFIOPCIDevice
indicates that all vectors from 0 to nr_vectors are allocated (and may
be enabled), which is used to to loop all the possibly used vectors
When, e.g., disabling MSI-X interrupts.

Extend the vector_use function to support dynamic MSI-X allocation when
host supports the capability. Qemu therefore can individually allocate
and enable a new interrupt without affecting others or causing interrupts
lost during runtime.

Utilize nr_vectors to calculate the upper bound of enabled vectors in
dynamic MSI-X allocation mode since looping all msix_entries_nr is not
efficient and unnecessary.

Signed-off-by: Jing Liu <jing2.liu@intel.com>
Tested-by: Reinette Chatre <reinette.chatre@intel.com>
---
 hw/vfio/pci.c | 40 +++++++++++++++++++++++++++-------------
 1 file changed, 27 insertions(+), 13 deletions(-)

diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
index 0c4ac0873d40..8c485636445c 100644
--- a/hw/vfio/pci.c
+++ b/hw/vfio/pci.c
@@ -512,12 +512,20 @@ static int vfio_msix_vector_do_use(PCIDevice *pdev, unsigned int nr,
     }
 
     /*
-     * We don't want to have the host allocate all possible MSI vectors
-     * for a device if they're not in use, so we shutdown and incrementally
-     * increase them as needed.
+     * When dynamic allocation is not supported, we don't want to have the
+     * host allocate all possible MSI vectors for a device if they're not
+     * in use, so we shutdown and incrementally increase them as needed.
+     * And nr_vectors stands for the number of vectors being allocated.
+     *
+     * When dynamic allocation is supported, let the host only allocate
+     * and enable a vector when it is in use in guest. nr_vectors stands
+     * for the upper bound of vectors being enabled (but not all of the
+     * ranges is allocated or enabled).
      */
-    if (vdev->nr_vectors < nr + 1) {
+    if ((vdev->msix->irq_info_flags & VFIO_IRQ_INFO_NORESIZE) &&
+        (vdev->nr_vectors < nr + 1)) {
         vdev->nr_vectors = nr + 1;
+
         if (!vdev->defer_kvm_irq_routing) {
             vfio_disable_irqindex(&vdev->vbasedev, VFIO_PCI_MSIX_IRQ_INDEX);
             ret = vfio_enable_vectors(vdev, true);
@@ -529,16 +537,22 @@ static int vfio_msix_vector_do_use(PCIDevice *pdev, unsigned int nr,
         Error *err = NULL;
         int32_t fd;
 
-        if (vector->virq >= 0) {
-            fd = event_notifier_get_fd(&vector->kvm_interrupt);
-        } else {
-            fd = event_notifier_get_fd(&vector->interrupt);
-        }
+        if (!vdev->defer_kvm_irq_routing) {
+            if (vector->virq >= 0) {
+                fd = event_notifier_get_fd(&vector->kvm_interrupt);
+            } else {
+                fd = event_notifier_get_fd(&vector->interrupt);
+            }
 
-        if (vfio_set_irq_signaling(&vdev->vbasedev,
-                                     VFIO_PCI_MSIX_IRQ_INDEX, nr,
-                                     VFIO_IRQ_SET_ACTION_TRIGGER, fd, &err)) {
-            error_reportf_err(err, VFIO_MSG_PREFIX, vdev->vbasedev.name);
+            if (vfio_set_irq_signaling(&vdev->vbasedev,
+                                       VFIO_PCI_MSIX_IRQ_INDEX, nr,
+                                       VFIO_IRQ_SET_ACTION_TRIGGER, fd, &err)) {
+                error_reportf_err(err, VFIO_MSG_PREFIX, vdev->vbasedev.name);
+            }
+        }
+        /* Increase for dynamic allocation case. */
+        if (vdev->nr_vectors < nr + 1) {
+            vdev->nr_vectors = nr + 1;
         }
     }
 
-- 
2.27.0