vfio: Move vfio_reset_handler() into device.c

Pass-through devices of a VM are not necessarily in the same group and
all groups/address_spaces need to be scanned when the machine is
reset. Commit f16f39c3fc ("Implement PCI hot reset") introduced a VM
reset handler for this purpose. Move it under device.c

Also reintroduce the comment which explained the context and was lost
along the way.

Reviewed-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: John Levon <john.levon@nutanix.com>
Link: https://lore.kernel.org/qemu-devel/20250326075122.1299361-26-clg@redhat.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>

hw/vfio/common.c

@@ -1300,21 +1300,3 @@ const MemoryListener vfio_memory_listener = {
     .log_global_stop = vfio_listener_log_global_stop,
     .log_sync = vfio_listener_log_sync,
 };
-
-void vfio_reset_handler(void *opaque)
-{
-    VFIODevice *vbasedev;
-
-    trace_vfio_reset_handler();
-    QLIST_FOREACH(vbasedev, &vfio_device_list, global_next) {
-        if (vbasedev->dev->realized) {
-            vbasedev->ops->vfio_compute_needs_reset(vbasedev);
-        }
-    }
-
-    QLIST_FOREACH(vbasedev, &vfio_device_list, global_next) {
-        if (vbasedev->dev->realized && vbasedev->needs_reset) {
-            vbasedev->ops->vfio_hot_reset_multi(vbasedev);
-        }
-    }
-}

hw/vfio/device.c

@@ -34,6 +34,41 @@
 VFIODeviceList vfio_device_list =
     QLIST_HEAD_INITIALIZER(vfio_device_list);
 
+/*
+ * We want to differentiate hot reset of multiple in-use devices vs
+ * hot reset of a single in-use device. VFIO_DEVICE_RESET will already
+ * handle the case of doing hot resets when there is only a single
+ * device per bus. The in-use here refers to how many VFIODevices are
+ * affected. A hot reset that affects multiple devices, but only a
+ * single in-use device, means that we can call it from our bus
+ * ->reset() callback since the extent is effectively a single
+ * device. This allows us to make use of it in the hotplug path. When
+ * there are multiple in-use devices, we can only trigger the hot
+ * reset during a system reset and thus from our reset handler. We
+ * separate _one vs _multi here so that we don't overlap and do a
+ * double reset on the system reset path where both our reset handler
+ * and ->reset() callback are used. Calling _one() will only do a hot
+ * reset for the one in-use devices case, calling _multi() will do
+ * nothing if a _one() would have been sufficient.
+ */
+void vfio_reset_handler(void *opaque)
+{
+    VFIODevice *vbasedev;
+
+    trace_vfio_reset_handler();
+    QLIST_FOREACH(vbasedev, &vfio_device_list, global_next) {
+        if (vbasedev->dev->realized) {
+            vbasedev->ops->vfio_compute_needs_reset(vbasedev);
+        }
+    }
+
+    QLIST_FOREACH(vbasedev, &vfio_device_list, global_next) {
+        if (vbasedev->dev->realized && vbasedev->needs_reset) {
+            vbasedev->ops->vfio_hot_reset_multi(vbasedev);
+        }
+    }
+}
+
 /*
  * Common VFIO interrupt disable
  */

hw/vfio/trace-events

@@ -108,7 +108,6 @@ vfio_put_base_device(int fd) "close vdev->fd=%d"
 vfio_legacy_dma_unmap_overflow_workaround(void) ""
 vfio_get_dirty_bitmap(uint64_t iova, uint64_t size, uint64_t bitmap_size, uint64_t start, uint64_t dirty_pages) "iova=0x%"PRIx64" size= 0x%"PRIx64" bitmap_size=0x%"PRIx64" start=0x%"PRIx64" dirty_pages=%"PRIu64
 vfio_iommu_map_dirty_notify(uint64_t iova_start, uint64_t iova_end) "iommu dirty @ 0x%"PRIx64" - 0x%"PRIx64
-vfio_reset_handler(void) ""
 
 # region.c
 vfio_region_write(const char *name, int index, uint64_t addr, uint64_t data, unsigned size) " (%s:region%d+0x%"PRIx64", 0x%"PRIx64 ", %d)"
@@ -196,3 +195,4 @@ iommufd_cdev_pci_hot_reset_dep_devices(int domain, int bus, int slot, int functi
 
 # device.c
 vfio_get_dev_region(const char *name, int index, uint32_t type, uint32_t subtype) "%s index %d, %08x/%08x"
+vfio_reset_handler(void) ""