tap: reset vnet header size on open

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 <[email protected]>

---

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(+)

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 <[email protected]>

---

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(+)

net/tap.c +7 -0

index df89caa..1abfd44 100644
@@ -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;

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