[PATCH 19/40] vdpa: should avoid map flushing with persistent iotlb

[Si-Wei Liu]

Today memory listener is unregistered in vhost_vdpa_reset_status
unconditionally, due to which all the maps will be flushed away
from the iotlb. However, map flush is not always needed, and
doing it from performance hot path may have innegligible latency
impact that affects VM reboot time or brown out period during
live migration.

Leverage the IOTLB_PERSIST backend featuae, which ensures durable
iotlb maps and not disappearing even across reset. When it is
supported, we may conditionally keep the maps for cases where the
guest memory mapping doesn't change. Prepare a function so that
the next patch will be able to use it to keep the maps.

Signed-off-by: Si-Wei Liu <si-wei.liu@oracle.com>
---
 hw/virtio/trace-events |  1 +
 hw/virtio/vhost-vdpa.c | 20 ++++++++++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/hw/virtio/trace-events b/hw/virtio/trace-events
index 77905d1..9725d44 100644
--- a/hw/virtio/trace-events
+++ b/hw/virtio/trace-events
@@ -66,6 +66,7 @@ vhost_vdpa_set_owner(void *dev) "dev: %p"
 vhost_vdpa_vq_get_addr(void *dev, void *vq, uint64_t desc_user_addr, uint64_t 
avail_user_addr, uint64_t used_user_addr) "dev: %p vq: %p desc_user_addr: 
0x%"PRIx64" avail_user_addr: 0x%"PRIx64" used_user_addr: 0x%"PRIx64
 vhost_vdpa_get_iova_range(void *dev, uint64_t first, uint64_t last) "dev: %p 
first: 0x%"PRIx64" last: 0x%"PRIx64
 vhost_vdpa_set_config_call(void *dev, int fd)"dev: %p fd: %d"
+vhost_vdpa_maybe_flush_map(void *dev, bool reg, bool flush, bool persist) 
"dev: %p registered: %d flush_map: %d iotlb_persistent: %d"
 
 # virtio.c
 virtqueue_alloc_element(void *elem, size_t sz, unsigned in_num, unsigned 
out_num) "elem %p size %zd in_num %u out_num %u"
diff --git a/hw/virtio/vhost-vdpa.c b/hw/virtio/vhost-vdpa.c
index ea2dfc8..31e0a55 100644
--- a/hw/virtio/vhost-vdpa.c
+++ b/hw/virtio/vhost-vdpa.c
@@ -1471,6 +1471,26 @@ out_stop:
     return ok ? 0 : -1;
 }
 
+static void vhost_vdpa_maybe_flush_map(struct vhost_dev *dev)
+{
+    struct vhost_vdpa *v = dev->opaque;
+
+    trace_vhost_vdpa_maybe_flush_map(dev, v->shared->listener_registered,
+                                     v->shared->flush_map,
+                                     !!(dev->backend_cap &
+                                     BIT_ULL(VHOST_BACKEND_F_IOTLB_PERSIST)));
+
+    if (!v->shared->listener_registered) {
+        return;
+    }
+
+    if (!(dev->backend_cap & BIT_ULL(VHOST_BACKEND_F_IOTLB_PERSIST)) ||
+        v->shared->flush_map) {
+        memory_listener_unregister(&v->shared->listener);
+        v->shared->listener_registered = false;
+    }
+}
+
 static void vhost_vdpa_reset_status(struct vhost_dev *dev)
 {
     struct vhost_vdpa *v = dev->opaque;
-- 
1.8.3.1