virtio: move allocation to virtqueue_pop/vring_pop

The return code of virtqueue_pop/vring_pop is unused except to check for
errors or 0.  We can thus easily move allocation inside the functions
and just return a pointer to the VirtQueueElement.

The advantage is that we will be able to allocate only the space that
is needed for the actual size of the s/g list instead of the full
VIRTQUEUE_MAX_SIZE items.  Currently VirtQueueElement takes about 48K
of memory, and this kind of allocation puts a lot of stress on malloc.
By cutting the size by two or three orders of magnitude, malloc can
use much more efficient algorithms.

The patch is pretty large, but changes to each device are testable
more or less independently.  Splitting it would mostly add churn.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>

2 files changed, 11 insertions, 15 deletions

diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
index ee0c4d4070..0d9978109c 100644
--- a/hw/block/dataplane/virtio-blk.c
+++ b/hw/block/dataplane/virtio-blk.c
@@ -100,20 +100,19 @@ static void handle_notify(EventNotifier *e)
     blk_io_plug(s->conf->conf.blk);
     for (;;) {
         MultiReqBuffer mrb = {};
-        int ret;
 
         /* Disable guest->host notifies to avoid unnecessary vmexits */
         vring_disable_notification(s->vdev, &s->vring);
 
         for (;;) {
-            VirtIOBlockReq *req = virtio_blk_alloc_request(vblk);
+            VirtIOBlockReq *req = vring_pop(s->vdev, &s->vring,
+                                            sizeof(VirtIOBlockReq));
 
-            ret = vring_pop(s->vdev, &s->vring, &req->elem);
-            if (ret < 0) {
-                virtio_blk_free_request(req);
+            if (req == NULL) {
                 break; /* no more requests */
             }
 
+            virtio_blk_init_request(vblk, req);
             trace_virtio_blk_data_plane_process_request(s, req->elem.out_num,
                                                         req->elem.in_num,
                                                         req->elem.index);
@@ -125,7 +124,7 @@ static void handle_notify(EventNotifier *e)
             virtio_blk_submit_multireq(s->conf->conf.blk, &mrb);
         }
 
-        if (likely(ret == -EAGAIN)) { /* vring emptied */
+        if (likely(!vring_more_avail(s->vdev, &s->vring))) { /* vring emptied */
             /* Re-enable guest->host notifies and stop processing the vring.
              * But if the guest has snuck in more descriptors, keep processing.
              */
diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
index 11bedff6d6..bf70b52dfd 100644
--- a/hw/block/virtio-blk.c
+++ b/hw/block/virtio-blk.c
@@ -29,15 +29,13 @@
 #include "hw/virtio/virtio-bus.h"
 #include "hw/virtio/virtio-access.h"
 
-VirtIOBlockReq *virtio_blk_alloc_request(VirtIOBlock *s)
+void virtio_blk_init_request(VirtIOBlock *s, VirtIOBlockReq *req)
 {
-    VirtIOBlockReq *req = g_new(VirtIOBlockReq, 1);
     req->dev = s;
     req->qiov.size = 0;
     req->in_len = 0;
     req->next = NULL;
     req->mr_next = NULL;
-    return req;
 }
 
 void virtio_blk_free_request(VirtIOBlockReq *req)
@@ -193,13 +191,11 @@ out:
 
 static VirtIOBlockReq *virtio_blk_get_request(VirtIOBlock *s)
 {
-    VirtIOBlockReq *req = virtio_blk_alloc_request(s);
+    VirtIOBlockReq *req = virtqueue_pop(s->vq, sizeof(VirtIOBlockReq));
 
-    if (!virtqueue_pop(s->vq, &req->elem)) {
-        virtio_blk_free_request(req);
-        return NULL;
+    if (req) {
+        virtio_blk_init_request(s, req);
     }
-
     return req;
 }
 
@@ -836,7 +832,8 @@ static int virtio_blk_load_device(VirtIODevice *vdev, QEMUFile *f,
     VirtIOBlock *s = VIRTIO_BLK(vdev);
 
     while (qemu_get_sbyte(f)) {
-        VirtIOBlockReq *req = virtio_blk_alloc_request(s);
+        VirtIOBlockReq *req = g_new(VirtIOBlockReq, 1);
+        virtio_blk_init_request(s, req);
         qemu_get_buffer(f, (unsigned char *)&req->elem,
                         sizeof(VirtQueueElement));
         req->next = s->rq;