[Qemu-devel] [PATCHv2] tap: reset vnet header size on open

[Qemu-devel] [PATCHv2] tap: reset vnet header size on open

[Michael S. Tsirkin]

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 host header size in tap device 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.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/net/tap.c b/net/tap.c
index df89caa..1abfd44 100644
--- a/net/tap.c
+++ b/net/tap.c
@@ -341,6 +341,13 @@ static TAPState *net_tap_fd_init(NetClientState *peer,
     s->using_vnet_hdr = 0;
     s->has_ufo = tap_probe_has_ufo(s->fd);
     tap_set_offload(&s->nc, 0, 0, 0, 0, 0);
+    /*
+     * Make sure host header length is set correctly in tap:
+     * it might have been modified by another instance of qemu.
+     */
+    if (tap_probe_vnet_hdr_len(s->fd, s->host_vnet_hdr_len)) {
+        tap_fd_set_vnet_hdr_len(s->fd, s->host_vnet_hdr_len);
+    }
     tap_read_poll(s, 1);
     s->vhost_net = NULL;
     return s;
-- 
MST

Re: [Qemu-devel] [PATCHv2] tap: reset vnet header size on open

[Stefan Hajnoczi]

On Tue, Nov 13, 2012 at 12:23:23PM +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 host header size in tap device 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.c | 7 +++++++
>  1 file changed, 7 insertions(+)

Thanks, applied to the net tree:
https://github.com/stefanha/qemu/commits/net

Stefan

Re: [Qemu-devel] [PATCHv2] tap: reset vnet header size on open

[Alexander Graf]

On 13.11.2012, at 11:23, 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 host header size in tap device 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?

Yup, works for me :).

Tested-by: Alexander Graf <agraf@suse.de>


Alex

> Thanks,
> 
> net/tap.c | 7 +++++++
> 1 file changed, 7 insertions(+)
> 
> diff --git a/net/tap.c b/net/tap.c
> index df89caa..1abfd44 100644
> --- a/net/tap.c
> +++ b/net/tap.c
> @@ -341,6 +341,13 @@ static TAPState *net_tap_fd_init(NetClientState *peer,
>     s->using_vnet_hdr = 0;
>     s->has_ufo = tap_probe_has_ufo(s->fd);
>     tap_set_offload(&s->nc, 0, 0, 0, 0, 0);
> +    /*
> +     * Make sure host header length is set correctly in tap:
> +     * it might have been modified by another instance of qemu.
> +     */
> +    if (tap_probe_vnet_hdr_len(s->fd, s->host_vnet_hdr_len)) {
> +        tap_fd_set_vnet_hdr_len(s->fd, s->host_vnet_hdr_len);
> +    }
>     tap_read_poll(s, 1);
>     s->vhost_net = NULL;
>     return s;
> -- 
> MST