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Date: Mon, 12 Nov 2012 13:23:39 +0100
From: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20121112122339.GC32040@stefanha-thinkpad.redhat.com>
References: <20121112071918.GA31752@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] tap: reset vnet header size on open
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To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Anthony Liguori <aliguori@us.ibm.com>, qemu-devel@nongnu.org, agraf@suse.de

On Mon, Nov 12, 2012 at 09:19:18AM +0200, Michael S. Tsirkin wrote:
> For tap, we currently assume the vnet header size is 10
> (the default value) but that might not be the case
> if tap is persistent and has been used by qemu previously.
> To fix, set vnet header size correctly on open.
> 
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
> 
> This fixes the issue reported by Alexander Graf on the kvm forum.
> Alexander, could you confirm please?
> Thanks,
> 
>  net/tap-linux.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/net/tap-linux.c b/net/tap-linux.c
> index c6521be..3eaedc4 100644
> --- a/net/tap-linux.c
> +++ b/net/tap-linux.c
> @@ -39,6 +39,7 @@ int tap_open(char *ifname, int ifname_size, int *vnet_hdr, int vnet_hdr_required
>  {
>      struct ifreq ifr;
>      int fd, ret;
> +    int len = sizeof(struct virtio_net_hdr);

We probably want the same behavior in the tap,fd= codepath.  How about a
call to tap_set_vnet_hdr_len() in net_init_tap() instead?

Stefan