Re: [Qemu-devel] [PATCH] net: Add persistent flag to -net tap option

[Jason Wang <jasowang@redhat.com>]

On 12/29/2014 03:38 PM, Roy Vardi wrote:
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>  
>
>  
>
> > -----Original Message-----
>
> > From: Jason Wang [mailto:jasowang@redhat.com]
>
> > Sent: Tuesday, December 23, 2014 11:13 AM
>
> > To: Roy Vardi
>
> > Cc: qemu-devel@nongnu.org; stefanha@redhat.com; Noam Camus;
>
> > armbru@redhat.com; aliguori@amazon.com; lcapitulino@redhat.com
>
> > Subject: Re: [Qemu-devel] [PATCH] net: Add persistent flag to -net
> tap option
>
> >
>
> >
>
> >
>
> > On Tue, Dec 23, 2014 at 4:44 PM, Roy Vardi <royv@ezchip.com
> <mailto:royv@ezchip.com>> wrote:
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> > >
>
> > >
>
> > >>  -----Original Message-----
>
> > >>  From: Jason Wang [mailto:jasowang@redhat.com]
>
> > >>  Sent: Monday, December 22, 2014 8:33 AM
>
> > >>  To: Roy Vardi; qemu-devel@nongnu.org <mailto:qemu-devel@nongnu.org>
>
> > >>  Cc: aliguori@amazon.com <mailto:aliguori@amazon.com>;
> armbru@redhat.com <mailto:armbru@redhat.com>; lcapitulino@redhat.com
> <mailto:lcapitulino@redhat.com>;
>
> > >> Noam Camus; stefanha@redhat.com <mailto:stefanha@redhat.com>
>
> > >>  Subject: Re: [Qemu-devel] [PATCH] net: Add persistent flag to -net
>
> > >> tap option
>
> > >>
>
> > >>
>
> > >>  On 12/21/2014 03:48 PM, Roy Vardi wrote:
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> > >>  > From: Roy Vardi <royv@ezchip.com <mailto:royv@ezchip.com>>
>
> > >>  >
>
> > >>  >     Add 'persistent' boolean flag to -net tap option.
>
> > >>  >     When set to off - tap interface will be released on shutdown
>
> > >>  >     When set to on\not specified - tap interface will remain
>
> > >>
>
> > >>  I'm interested of the user cases in the case. Usually, persistent
>
> > >> flag was used to  let privileged application to create/configure the
>
> > >> device and then it could be  used by non-privileged users. If qemu
>
> > >> has already had privilege, why need set  persistent in this case?
>
> > >
>
> > > We're running qemu as users, non-privilege...
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> > > Our work flow includes:
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> > > 1. Running an internal tool for opening a persistent tap interface 2.
>
> > > Running qemu with the tap interface we got from above Our environment
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> > > includes a lot of such qemu runs, and we want to avoid "zombie" tap
>
> > > interfaces, and we achieve it with this new flag I've added.
>
> >
>
> > I get the case, thanks for the explaining. But qemu has already had
> method to
>
> > solve this. Try downscript for tap, this external script can do
> cleanup before
>
> > closing tap fd.
>
> >
>
> > E.g. in your case, you may need to run tunctl -d.
>
>  
>
> Thanks for the reference.
>
> I've checked the downscript option, but found it unsuitable for us:
> Qemu runs the downscript before closing the fd, so a script which
> removes the interface will fail due to busy device.
>

Right, tunctl needs another TUNSETIFF ioctl(). You may want to delete it
through ip link del link dev $1 in your qemu-ifdown.
>
> I've changed the order in the qemu code so that the script is called
> after the descriptor is closed and it works for us.
>
>  
>
> What do you think about the following patch:
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>  
>

This change behaviour which may break existing down scripts.

Thanks
>
> --- a/net/tap.c
>
> +++ b/net/tap.c
>
> @@ -296,12 +296,11 @@ static void tap_cleanup(NetClientState *nc)
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>      qemu_purge_queued_packets(nc);
>
> -    if (s->down_script[0])
>
> -        launch_script(s->down_script, s->down_script_arg, s->fd);
>
> -
>
>      tap_read_poll(s, false);
>
>      tap_write_poll(s, false);
>
>      close(s->fd);
>
> +    if (s->down_script[0])
>
> +        launch_script(s->down_script, s->down_script_arg, s->fd);
>
>      s->fd = -1;
>
> }
>
>  
>
> ?
>
>  
>
> Thanks,
>
> Roy Vardi
>