util: stop probing for IFF_VNET_HDR

This flag was added by Linux with:

  commit f43798c27684ab925adde7d8acc34c78c6e50df8
  Author: Rusty Russell <rusty@rustcorp.com.au>
  Date:   Thu Jul 3 03:48:02 2008 -0700

    tun: Allow GSO using virtio_net_hdr

so we can assume all Linux distros we support have this flag available
and thus the compile time check is sufficient.

Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

src/util/virnetdevtap.c

@@ -183,66 +183,6 @@ virNetDevTapGetRealDeviceName(char *ifname G_GNUC_UNUSED)
 }
 
 
-/**
- * virNetDevProbeVnetHdr:
- * @tapfd: a tun/tap file descriptor
- *
- * Check whether it is safe to enable the IFF_VNET_HDR flag on the
- * tap interface.
- *
- * Setting IFF_VNET_HDR enables QEMU's virtio_net driver to allow
- * guests to pass larger (GSO) packets, with partial checksums, to
- * the host. This greatly increases the achievable throughput.
- *
- * It is only useful to enable this when we're setting up a virtio
- * interface. And it is only *safe* to enable it when we know for
- * sure that a) qemu has support for IFF_VNET_HDR and b) the running
- * kernel implements the TUNGETIFF ioctl(), which qemu needs to query
- * the supplied tapfd.
- *
- * Returns 1 if VnetHdr is supported, 0 if not supported
- */
-#ifdef IFF_VNET_HDR
-static int
-virNetDevProbeVnetHdr(int tapfd)
-{
-# if defined(IFF_VNET_HDR) && defined(TUNGETFEATURES) && defined(TUNGETIFF)
-    unsigned int features;
-    struct ifreq dummy;
-
-    if (ioctl(tapfd, TUNGETFEATURES, &features) != 0) {
-        VIR_INFO("Not enabling IFF_VNET_HDR; "
-                 "TUNGETFEATURES ioctl() not implemented");
-        return 0;
-    }
-
-    if (!(features & IFF_VNET_HDR)) {
-        VIR_INFO("Not enabling IFF_VNET_HDR; "
-                 "TUNGETFEATURES ioctl() reports no IFF_VNET_HDR");
-        return 0;
-    }
-
-    /* The kernel will always return -1 at this point.
-     * If TUNGETIFF is not implemented then errno == EBADFD.
-     */
-    if (ioctl(tapfd, TUNGETIFF, &dummy) != -1 || errno != EBADFD) {
-        VIR_INFO("Not enabling IFF_VNET_HDR; "
-                 "TUNGETIFF ioctl() not implemented");
-        return 0;
-    }
-
-    VIR_INFO("Enabling IFF_VNET_HDR");
-
-    return 1;
-# else
-    (void) tapfd;
-    VIR_INFO("Not enabling IFF_VNET_HDR; disabled at build time");
-    return 0;
-# endif
-}
-#endif
-
-
 #ifdef TUNSETIFF
 /**
  * virNetDevTapGenerateName:
@@ -363,8 +303,7 @@ int virNetDevTapCreate(char **ifname,
         }
 
 # ifdef IFF_VNET_HDR
-        if ((flags &  VIR_NETDEV_TAP_CREATE_VNET_HDR) &&
+        if (flags &  VIR_NETDEV_TAP_CREATE_VNET_HDR)
-            virNetDevProbeVnetHdr(fd))
             ifr.ifr_flags |= IFF_VNET_HDR;
 # endif