virtio: limit avail bytes lookahead

Commit 0d8d7690850eb0cf2b2b60933cf47669a6b6f18f 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: Michael S. Tsirkin <mst@redhat.com>
Reported-by: Edivaldo de Araujo Pereira <edivaldoapereira@yahoo.com.br>
Tested-by: Edivaldo de Araujo Pereira <edivaldoapereira@yahoo.com.br>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

1 files changed, 9 insertions, 3 deletions

diff --git a/hw/virtio-rng.c b/hw/virtio-rng.c
index df329f2..a73ef8e 100644
--- a/hw/virtio-rng.c
+++ b/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);