File kvm-qemu-preXX-net-fix-qemu_can_send_packet-logic.patch of Package kvm.853

From 60c07d933c66c4b30a83b7ccbc8a0cb3df1b2d0e Mon Sep 17 00:00:00 2001
From: Vincent Palatin <>
Date: Wed, 2 Mar 2011 17:25:02 -0500
Subject: [PATCH] net: fix qemu_can_send_packet logic

If any of the clients is not ready to receive (ie it has a can_receive
callback and can_receive() returns false), we don't want to start
sending, else this client may miss/discard the packet.

I got this behaviour with the following setup :
the emulated machine is using an USB-ethernet adapter, it is connected
to the network using SLIRP and I'm dumping the traffic in a .pcap file.
As per the following command line :
-net nic,model=usb,vlan=1 -net user,vlan=1 -net dump,vlan=1,file=/tmp/pkt.pcap
Every time that two packets are coming in a row from the host, the
usb-net code will receive the first one, then returns 0 to can_receive
call since it has a 1 packet long queue. But as the dump code is always
ready to receive, qemu_can_send_packet will return true and the next
packet will discard the previous one in the usb-net code.

Signed-off-by: Vincent Palatin <>
Signed-off-by: Blue Swirl <>
 net.c |    6 +++---
 1 files changed, 3 insertions(+), 3 deletions(-)

Index: qemu-kvm-0.14.0/net.c
--- qemu-kvm-0.14.0.orig/net.c
+++ qemu-kvm-0.14.0/net.c
@@ -432,11 +432,11 @@ int qemu_can_send_packet(VLANClientState
         /* no can_receive() handler, they can always receive */
-        if (!vc->info->can_receive || vc->info->can_receive(vc)) {
-            return 1;
+        if (vc->info->can_receive && !vc->info->can_receive(vc)) {
+            return 0;
-    return 0;
+    return 1;
 static ssize_t qemu_deliver_packet(VLANClientState *sender,