[PATCH] kvm: qemu: Handle tap fds with IFF_VNET_HDR

[Mark McLoughlin <markmc@redhat.com>]

Allow users to pass an IFF_VNET_HDR tap fd via "-net tap,fd=X"
by querying the fd with the recently added TUNGETIFF ioctl() to
see if IFF_VNET_HDR has been enabled.

Note: users wishing to pass an IFF_VNET_HDR tap fd to qemu
should check both that a) this version of qemu can handle
such an fd and b) that the TUNGETIFF ioctl() is available in
the running kernel.

We add a comment to the "qemu -help" output to indicate to
users that this version of qemu supports IFF_VNET_HDR.

Signed-off-by: Mark McLoughlin <markmc@redhat.com>
---
 qemu/vl.c |   21 ++++++++++++++++++++-
 1 files changed, 20 insertions(+), 1 deletions(-)

diff --git a/qemu/vl.c b/qemu/vl.c
index 2dc1311..b9b202e 100644
--- a/qemu/vl.c
+++ b/qemu/vl.c
@@ -4536,6 +4536,22 @@ void tap_using_vnet_hdr(void *opaque, int using_vnet_hdr)
     s->using_vnet_hdr = using_vnet_hdr != 0;
 }
 
+static int tap_probe_vnet_hdr(int fd)
+{
+#if defined(TUNGETIFF) && defined(IFF_VNET_HDR)
+    struct ifreq ifr;
+
+    if (ioctl(fd, TUNGETIFF, &ifr) != 0) {
+	fprintf(stderr, "TUNGETIFF ioctl() failed: %s\n", strerror(errno));
+	return 0;
+    }
+
+    return ifr.ifr_flags & IFF_VNET_HDR;
+#else
+    return 0;
+#endif
+}
+
 #ifdef TUNSETOFFLOAD
 static void tap_set_offload(VLANClientState *vc, int csum, int tso4, int tso6,
 			    int ecn)
@@ -5501,7 +5517,7 @@ int net_client_init(const char *device, const char *p)
             fd = strtol(buf, NULL, 0);
             fcntl(fd, F_SETFL, O_NONBLOCK);
             ret = -1;
-            if (net_tap_fd_init(vlan, fd, 0))
+            if (net_tap_fd_init(vlan, fd, tap_probe_vnet_hdr(fd)))
                 ret = 0;
         } else {
             if (get_param_value(ifname, sizeof(ifname), "ifname", p) <= 0) {
@@ -8319,6 +8335,9 @@ static void help(int exitcode)
            "                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"
+#if defined(TUNGETIFF) && defined(IFF_VNET_HDR)
+           "                'h' may optinally be configured with the IFF_VNET_HDR flag\n"
+#endif
 #endif
            "-net socket[,vlan=n][,fd=h][,listen=[host]:port][,connect=host:port]\n"
            "                connect the vlan 'n' to another VLAN using a socket connection\n"
-- 
1.5.4.1