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

[Anthony Liguori <anthony@codemonkey.ws>]

Mark McLoughlin wrote:
> 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.
>   

The -help output is not a supported interface.  An info command in the 
monitor would be a better way to detect this.

Regards,

Anthony Liguori

> 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"
>