[PATCH 1/1] kvm: qemu: Handle tap fds with IFF_VNET_HDR

[Mark McLoughlin <markmc@redhat.com>]

On Tue, 2008-08-12 at 16:51 +0300, Avi Kivity wrote:
> Mark McLoughlin wrote:

> >         Lastly, I need to add a "vnet_hdr=on" param to "-net tap" so
> > that we can know if the supplied tap fd has IFF_VNET_HDR enabled.
> > There's no interface to query that on the fd right now, and even if
> > I did cook up a patch it probably wouldn't make 2.6.27.
> >   
> 
> except for this, as I agree with Anthony.  I think it can make it into 
> 2.6.27, but if it doesn't, libvirt should simply not enable vnet_hdr 
> unless it is sure qemu can query for the feature.

Okay, I've just sent a patch to add TUNGETIFF:

  http://marc.info/?l=linux-netdev&m=121863813904363

See below patch for how we'd use it. Don't apply this until the kernel
patch is accepted, obviously.

The -help string is what libvirt would use to detect whether qemu has
support for this; clearly libvirt would also need to detect the
availability of TUNGETIFF in the running kernel.

Cheers,
Mark.

Subject: [PATCH 1/1] kvm: qemu: Handle tap fds with IFF_VNET_HDR

Allow users to pass an IFF_VNET_HDR tap fd via "-net tap,fd=X"
by querying the fd with the newly 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