From b62ca8e7a52d6d18987d04e9c7cbbad3eef8023d Mon Sep 17 00:00:00 2001
From: Michael S. Tsirkin <mst@redhat.com>
Date: Wed, 2 Feb 2011 15:08:48 -0200
Subject: [PATCH 24/28] tap: safe sndbuf default
RH-Author: Michael S. Tsirkin <mst@redhat.com>
Message-id: <20110202150848.GA12274@redhat.com>
Patchwork-id: 17513
O-Subject: [PATCH resend RHEL6.1] tap: safe sndbuf default
Bugzilla: 674539
RH-Acked-by: Juan Quintela <quintela@redhat.com>
RH-Acked-by: Amit Shah <amit.shah@redhat.com>
RH-Acked-by: Jes Sorensen <Jes.Sorensen@redhat.com>
commit f157ed202e51dc2492b201dc34ed28e89c973fb7
Author: Michael S. Tsirkin <mst@redhat.com>
Date: Tue Feb 1 14:25:40 2011 +0200
tap: safe sndbuf default
With current sndbuf default value, a blocked
target guest can prevent another guest from
transmitting any packets. While current
sndbuf value (1M) is reported to help some
UDP based workloads, the default should
be safe (0).
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=674539
---
net/tap-linux.c | 13 +++++++++----
qemu-options.hx | 4 ++--
2 files changed, 11 insertions(+), 6 deletions(-)
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
---
net/tap-linux.c | 13 +++++++++----
qemu-options.hx | 4 ++--
2 files changed, 11 insertions(+), 6 deletions(-)
diff --git a/net/tap-linux.c b/net/tap-linux.c
index a425ff0..00f84d4 100644
--- a/net/tap-linux.c
+++ b/net/tap-linux.c
@@ -80,12 +80,17 @@ int tap_open(char *ifname, int ifname_size, int *vnet_hdr, int vnet_hdr_required
return fd;
}
-/* sndbuf should be set to a value lower than the tx queue
- * capacity of any destination network interface.
+/* sndbuf implements a kind of flow control for tap.
+ * Unfortunately when it's enabled, and packets are sent
+ * to other guests on the same host, the receiver
+ * can lock up the transmitter indefinitely.
+ *
+ * To avoid packet loss, sndbuf should be set to a value lower than the tx
+ * queue capacity of any destination network interface.
* Ethernet NICs generally have txqueuelen=1000, so 1Mb is
- * a good default, given a 1500 byte MTU.
+ * a good value, given a 1500 byte MTU.
*/
-#define TAP_DEFAULT_SNDBUF 1024*1024
+#define TAP_DEFAULT_SNDBUF 0
int tap_set_sndbuf(int fd, QemuOpts *opts)
{
diff --git a/qemu-options.hx b/qemu-options.hx
index 9178387..01a0a5b 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -913,8 +913,8 @@ DEF("net", HAS_ARG, QEMU_OPTION_net,
" and 'dfile' (default=%s);\n"
" use '[down]script=no' to disable script execution;\n"
" use 'fd=h' to connect to an already opened TAP interface\n"
- " use 'sndbuf=nbytes' to limit the size of the send buffer; the\n"
- " default of 'sndbuf=1048576' can be disabled using 'sndbuf=0'\n"
+ " use 'sndbuf=nbytes' to limit the size of the send buffer (the\n"
+ " default is disabled 'sndbuf=0' to enable flow control set 'sndbuf=1048576')\n"
" use vnet_hdr=off to avoid enabling the IFF_VNET_HDR tap flag; use\n"
" vnet_hdr=on to make the lack of IFF_VNET_HDR support an error condition\n"
" use vhost=on to enable experimental in kernel accelerator\n"
--
1.7.4.rc1.16.gd2f15e