virtio: limit avail bytes lookahead

Commit 0d8d7690 introduced
a regression in virtio-net performance because it looks
into the ring aggressively while we really only care
about a single packet worth of buffers.
Reported as bugzilla 1066055 in launchpad.

To fix, add parameters limiting lookahead, and
use in virtqueue_avail_bytes.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Reported-by: NEdivaldo de Araujo Pereira <edivaldoapereira@yahoo.com.br>
Tested-by: NEdivaldo de Araujo Pereira <edivaldoapereira@yahoo.com.br>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

hw/virtio-rng.c

@@ -43,11 +43,11 @@ static bool is_guest_ready(VirtIORNG *vrng)
     return false;
 }
 
-static size_t get_request_size(VirtQueue *vq)
+static size_t get_request_size(VirtQueue *vq, unsigned quota)
 {
     unsigned int in, out;
 
-    virtqueue_get_avail_bytes(vq, &in, &out);
+    virtqueue_get_avail_bytes(vq, &in, &out, quota, 0);
     return in;
 }
 
@@ -84,12 +84,18 @@ static void chr_read(void *opaque, const void *buf, size_t size)
 static void virtio_rng_process(VirtIORNG *vrng)
 {
     size_t size;
+    unsigned quota;
 
     if (!is_guest_ready(vrng)) {
         return;
     }
 
-    size = get_request_size(vrng->vq);
+    if (vrng->quota_remaining < 0) {
+        quota = 0;
+    } else {
+        quota = MIN((uint64_t)vrng->quota_remaining, (uint64_t)UINT32_MAX);
+    }
+    size = get_request_size(vrng->vq, quota);
     size = MIN(vrng->quota_remaining, size);
     if (size) {
         rng_backend_request_entropy(vrng->rng, size, chr_read, vrng);

hw/virtio-serial-bus.c

@@ -306,7 +306,7 @@ size_t virtio_serial_guest_ready(VirtIOSerialPort *port)
     if (use_multiport(port->vser) && !port->guest_connected) {
         return 0;
     }
-    virtqueue_get_avail_bytes(vq, &bytes, NULL);
+    virtqueue_get_avail_bytes(vq, &bytes, NULL, 4096, 0);
     return bytes;
 }
 

hw/virtio.c

@@ -336,7 +336,8 @@ static unsigned virtqueue_next_desc(hwaddr desc_pa,
 }
 
 void virtqueue_get_avail_bytes(VirtQueue *vq, unsigned int *in_bytes,
-                               unsigned int *out_bytes)
+                               unsigned int *out_bytes,
+                               unsigned max_in_bytes, unsigned max_out_bytes)
 {
     unsigned int idx;
     unsigned int total_bufs, in_total, out_total;
@@ -385,6 +386,9 @@ void virtqueue_get_avail_bytes(VirtQueue *vq, unsigned int *in_bytes,
             } else {
                 out_total += vring_desc_len(desc_pa, i);
             }
+            if (in_total >= max_in_bytes && out_total >= max_out_bytes) {
+                goto done;
+            }
         } while ((i = virtqueue_next_desc(desc_pa, i, max)) != max);
 
         if (!indirect)
@@ -392,6 +396,7 @@ void virtqueue_get_avail_bytes(VirtQueue *vq, unsigned int *in_bytes,
         else
             total_bufs++;
     }
+done:
     if (in_bytes) {
         *in_bytes = in_total;
     }
@@ -405,12 +410,8 @@ int virtqueue_avail_bytes(VirtQueue *vq, unsigned int in_bytes,
 {
     unsigned int in_total, out_total;
 
-    virtqueue_get_avail_bytes(vq, &in_total, &out_total);
-    if ((in_bytes && in_bytes < in_total)
-        || (out_bytes && out_bytes < out_total)) {
-        return 1;
-    }
-    return 0;
+    virtqueue_get_avail_bytes(vq, &in_total, &out_total, in_bytes, out_bytes);
+    return in_bytes <= in_total && out_bytes <= out_total;
 }
 
 void virtqueue_map_sg(struct iovec *sg, hwaddr *addr,

hw/virtio.h

@@ -150,7 +150,8 @@ int virtqueue_pop(VirtQueue *vq, VirtQueueElement *elem);
 int virtqueue_avail_bytes(VirtQueue *vq, unsigned int in_bytes,
                           unsigned int out_bytes);
 void virtqueue_get_avail_bytes(VirtQueue *vq, unsigned int *in_bytes,
-                               unsigned int *out_bytes);
+                               unsigned int *out_bytes,
+                               unsigned max_in_bytes, unsigned max_out_bytes);
 
 void virtio_notify(VirtIODevice *vdev, VirtQueue *vq);