[PATCH v2 2/2] vdpa: commit all host notifier MRs in a single MR transaction

["Longpeng(Mike)" via <qemu-devel@nongnu.org>]

From: Longpeng <longpeng2@huawei.com>

This allows the vhost-vdpa device to batch the setup of all its MRs of
host notifiers.

This significantly reduces the device starting time, e.g. the time spend
on setup the host notifier MRs reduce from 423ms to 32ms for a VM with
64 vCPUs and 3 vhost-vDPA generic devices[1] (64vq per device).

[1] https://www.mail-archive.com/qemu-devel@nongnu.org/msg921541.html

Signed-off-by: Longpeng <longpeng2@huawei.com>
---
 hw/virtio/vhost-vdpa.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/hw/virtio/vhost-vdpa.c b/hw/virtio/vhost-vdpa.c
index 7468e44b87..eb233cf08a 100644
--- a/hw/virtio/vhost-vdpa.c
+++ b/hw/virtio/vhost-vdpa.c
@@ -547,9 +547,18 @@ static void vhost_vdpa_host_notifiers_uninit(struct vhost_dev *dev, int n)
 {
     int i;
 
+    /*
+     * Pack all the changes to the memory regions in a single
+     * transaction to avoid a few updating of the address space
+     * topology.
+     */
+    memory_region_transaction_begin();
+
     for (i = dev->vq_index; i < dev->vq_index + n; i++) {
         vhost_vdpa_host_notifier_uninit(dev, i);
     }
+
+    memory_region_transaction_commit();
 }
 
 static void vhost_vdpa_host_notifiers_init(struct vhost_dev *dev)
@@ -562,16 +571,25 @@ static void vhost_vdpa_host_notifiers_init(struct vhost_dev *dev)
         return;
     }
 
+    /*
+     * Pack all the changes to the memory regions in a single
+     * transaction to avoid a few updating of the address space
+     * topology.
+     */
+    memory_region_transaction_begin();
+
     for (i = dev->vq_index; i < dev->vq_index + dev->nvqs; i++) {
         if (vhost_vdpa_host_notifier_init(dev, i)) {
             goto err;
         }
     }
 
+    memory_region_transaction_commit();
     return;
 
 err:
     vhost_vdpa_host_notifiers_uninit(dev, i - dev->vq_index);
+    memory_region_transaction_commit();
     return;
 }
 
-- 
2.23.0