Re: [PATCH] util: stop probing for IFF_VNET_HDR

On a Thursday in 2020, Daniel P. Berrangé wrote: > This flag was added by Linux with: > > commit f43798c27684ab925adde7d8acc34c78c6e50df8 > Author: Rusty Russell <rusty(a)rustcorp.com.au&gt; > 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(a)redhat.com&gt; > --- > 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(a)redhat.com&gt; 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 >