[Qemu-devel] [PATCH v3] net.c: Moved large array in nc_sendv_compat from the stack to the heap

[Nikos Filippakis <aesmade@gmail.com>]

Allocate array in nc_sendv_compat on the heap if it is large to reduce stack frame size, as stated in the BiteSizedTasks wiki page.

Signed-off-by: Nikos Filippakis <aesmade@gmail.com>
---
 net/net.c | 32 +++++++++++++++++++++++++-------
 1 file changed, 25 insertions(+), 7 deletions(-)

diff --git a/net/net.c b/net/net.c
index b0c832e..a9be287 100644
--- a/net/net.c
+++ b/net/net.c
@@ -709,23 +709,41 @@ ssize_t qemu_send_packet_raw(NetClientState *nc, const uint8_t *buf, int size)
 static ssize_t nc_sendv_compat(NetClientState *nc, const struct iovec *iov,
                                int iovcnt, unsigned flags)
 {
-    uint8_t buf[NET_BUFSIZE];
-    uint8_t *buffer;
-    size_t offset;
+    const size_t STACKBUF_SIZE = 2048;
+
+    uint8_t *buffer, *dynbuf = NULL;
+    uint8_t stackbuf[STACKBUF_SIZE];
+    size_t offset, iov_len;
+    ssize_t ret;
 
     if (iovcnt == 1) {
         buffer = iov[0].iov_base;
         offset = iov[0].iov_len;
     } else {
-        buffer = buf;
-        offset = iov_to_buf(iov, iovcnt, 0, buf, sizeof(buf));
+        iov_len = iov_size(iov, iovcnt);
+
+        if (iov_len <= STACKBUF_SIZE) {
+            buffer = stackbuf;
+        } else if (iov_len <= NET_BUFSIZE) {
+            buffer = dynbuf = g_new(uint8_t, iov_len);
+        } else {
+            errno = EINVAL;
+            return 0;
+        }
+
+        offset = iov_to_buf(iov, iovcnt, 0, buffer,
+                            NET_BUFSIZE * sizeof(uint8_t));
     }
 
     if (flags & QEMU_NET_PACKET_FLAG_RAW && nc->info->receive_raw) {
-        return nc->info->receive_raw(nc, buffer, offset);
+        ret = nc->info->receive_raw(nc, buffer, offset);
     } else {
-        return nc->info->receive(nc, buffer, offset);
+        ret = nc->info->receive(nc, buffer, offset);
     }
+
+    g_free(dynbuf);
+
+    return ret;
 }
 
 ssize_t qemu_deliver_packet_iov(NetClientState *sender,
-- 
1.9.1