[PATCH 1/2] Reduce vdpa initialization / startup overhead

[peili.dev@gmail.com]

From: Pei Li <peili.dev@gmail.com>

Currently, part of the vdpa initialization / startup process
needs to trigger many ioctls per vq, which is very inefficient
and causing unnecessary context switch between user mode and
kernel mode.

This patch creates an additional ioctl() command, namely
VHOST_VDPA_GET_VRING_GROUP_BATCH, that will batching
commands of VHOST_VDPA_GET_VRING_GROUP into a single
ioctl() call.

Signed-off-by: Pei Li <peili.dev@gmail.com>
---
 hw/virtio/vhost-vdpa.c                       | 31 +++++++++++++++-----
 include/standard-headers/linux/vhost_types.h |  3 ++
 linux-headers/linux/vhost.h                  |  7 +++++
 3 files changed, 33 insertions(+), 8 deletions(-)

diff --git a/hw/virtio/vhost-vdpa.c b/hw/virtio/vhost-vdpa.c
index bc6bad23d5..6d45ff8539 100644
--- a/hw/virtio/vhost-vdpa.c
+++ b/hw/virtio/vhost-vdpa.c
@@ -679,7 +679,8 @@ static int vhost_vdpa_set_backend_cap(struct vhost_dev *dev)
     uint64_t f = 0x1ULL << VHOST_BACKEND_F_IOTLB_MSG_V2 |
         0x1ULL << VHOST_BACKEND_F_IOTLB_BATCH |
         0x1ULL << VHOST_BACKEND_F_IOTLB_ASID |
-        0x1ULL << VHOST_BACKEND_F_SUSPEND;
+        0x1ULL << VHOST_BACKEND_F_SUSPEND |
+        0x1ULL << VHOST_BACKEND_F_IOCTL_BATCH;
     int r;
 
     if (vhost_vdpa_call(dev, VHOST_GET_BACKEND_FEATURES, &features)) {
@@ -731,14 +732,28 @@ static int vhost_vdpa_get_vq_index(struct vhost_dev *dev, int idx)
 
 static int vhost_vdpa_set_vring_ready(struct vhost_dev *dev)
 {
-    int i;
+    int i, nvqs = dev->nvqs;
+    uint64_t backend_features = dev->backend_cap;
+
     trace_vhost_vdpa_set_vring_ready(dev);
-    for (i = 0; i < dev->nvqs; ++i) {
-        struct vhost_vring_state state = {
-            .index = dev->vq_index + i,
-            .num = 1,
-        };
-        vhost_vdpa_call(dev, VHOST_VDPA_SET_VRING_ENABLE, &state);
+
+    if (!(backend_features & BIT_ULL(VHOST_BACKEND_F_IOCTL_BATCH))) {
+        for (i = 0; i < nvqs; ++i) {
+            struct vhost_vring_state state = {
+                .index = dev->vq_index + i,
+                .num = 1,
+            };
+            vhost_vdpa_call(dev, VHOST_VDPA_SET_VRING_ENABLE, &state);
+        }
+    } else {
+        struct vhost_vring_state states[nvqs + 1];
+        states[0].num = nvqs;
+        for (i = 1; i <= nvqs; ++i) {
+            states[i].index = dev->vq_index + i - 1;
+            states[i].num = 1;
+        }
+
+        vhost_vdpa_call(dev, VHOST_VDPA_SET_VRING_ENABLE_BATCH, &states[0]);
     }
     return 0;
 }
diff --git a/include/standard-headers/linux/vhost_types.h b/include/standard-headers/linux/vhost_types.h
index c41a73fe36..068d0e1ceb 100644
--- a/include/standard-headers/linux/vhost_types.h
+++ b/include/standard-headers/linux/vhost_types.h
@@ -164,4 +164,7 @@ struct vhost_vdpa_iova_range {
 /* Device can be suspended */
 #define VHOST_BACKEND_F_SUSPEND  0x4
 
+/* IOCTL requests can be batched */
+#define VHOST_BACKEND_F_IOCTL_BATCH 0x6
+
 #endif
diff --git a/linux-headers/linux/vhost.h b/linux-headers/linux/vhost.h
index f9f115a7c7..4c9ddd0a0e 100644
--- a/linux-headers/linux/vhost.h
+++ b/linux-headers/linux/vhost.h
@@ -180,4 +180,11 @@
  */
 #define VHOST_VDPA_SUSPEND		_IO(VHOST_VIRTIO, 0x7D)
 
+/* Batch version of VHOST_VDPA_SET_VRING_ENABLE
+ *
+ * Enable/disable the ring while batching the commands.
+ */
+#define VHOST_VDPA_SET_VRING_ENABLE_BATCH	_IOW(VHOST_VIRTIO, 0x7F, \
+					     struct vhost_vring_state)
+
 #endif
-- 
2.25.1