[PATCH] util: stop probing for IFF_VNET_HDR

[Ján Tomko]

On a Thursday in 2020, Daniel P. Berrangé wrote:
>This flag was added by Linux with:
>
>  commit f43798c27684ab925adde7d8acc34c78c6e50df8
>  Author: Rusty Russell <rusty at 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.
>
>Signed-off-by: Daniel P. Berrangé <berrange at redhat.com>
>---
> src/util/virnetdevtap.c | 63 +----------------------------------------
> 1 file changed, 1 insertion(+), 62 deletions(-)
>
>diff --git a/src/util/virnetdevtap.c b/src/util/virnetdevtap.c
>index cbce5c71b7..77c4d1c52c 100644
>--- a/src/util/virnetdevtap.c
>+++ b/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

The TUNSETIFF guard above (which was introduced in Linux eons ago)
seems to be enough according to our CI:
https://gitlab.com/janotomko/libvirt/-/pipelines/193942161

It builds even with the IFF_MULTI_QUEUE guard removed.
commit bbb009941efaece3898910a862f6d23aa55d6ba8
CommitDate: 2012-11-01 11:14:08 -0400
     tuntap: introduce multiqueue flags

But #ifdef removal is out of scope of this patch.

Reviewed-by: Ján Tomko <jtomko at redhat.com>

Jano

>-        if ((flags &  VIR_NETDEV_TAP_CREATE_VNET_HDR) &&
>-            virNetDevProbeVnetHdr(fd))
>+        if (flags &  VIR_NETDEV_TAP_CREATE_VNET_HDR)
>             ifr.ifr_flags |= IFF_VNET_HDR;
> # endif
>
>-- 
>2.26.2
>
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