Re: [RFC 2/4] tuntap: Publish tuntap maximum number of queues as module_param

[Pankaj Gupta <pagupta@redhat.com>]

> On 08/24/2014 07:14 PM, Michael S. Tsirkin wrote:
> > On Fri, Aug 22, 2014 at 07:52:22AM -0400, Pankaj Gupta wrote:
> >>> On 08/20/2014 07:17 PM, Michael S. Tsirkin wrote:
> >>>> On Wed, Aug 20, 2014 at 12:58:17PM +0200, Jiri Pirko wrote:
> >>>>>> Mon, Aug 18, 2014 at 03:37:18PM CEST, pagupta@redhat.com wrote:
> >>>>>>>> This patch publishes maximum number of tun/tap queues allocated as a
> >>>>>>>> read_only module parameter which a user space application like
> >>>>>>>> libvirt
> >>>>>>>> can make use of to limit maximum number of queues. Value of
> >>>>>>>> read_only
> >>>>>>>> module parameter can be writable only at module load time. If no
> >>>>>>>> value is set
> >>>>>>>> at module load time a default value 256 is used which is equal to
> >>>>>>>> maximum number
> >>>>>>>> of vCPUS allowed by KVM.
> >>>>>>>>
> >>>>>>>> Administrator can specify maximum number of queues only at the
> >>>>>>>> driver
> >>>>>>>> module load time.
> >>>>>>>>
> >>>>>>>> Signed-off-by: Pankaj Gupta <pagupta@redhat.com>
> >>>>>>>> ---
> >>>>>>>> drivers/net/tun.c |   13 +++++++++++--
> >>>>>>>> 1 files changed, 11 insertions(+), 2 deletions(-)
> >>>>>>>>
> >>>>>>>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> >>>>>>>> index acaaf67..1f518e2 100644
> >>>>>>>> --- a/drivers/net/tun.c
> >>>>>>>> +++ b/drivers/net/tun.c
> >>>>>>>> @@ -119,6 +119,9 @@ struct tap_filter {
> >>>>>>>>
> >>>>>>>> #define TUN_FLOW_EXPIRE (3 * HZ)
> >>>>>>>>
> >>>>>>>> +static int max_tap_queues = MAX_TAP_QUEUES;
> >>>>>>>> +module_param(max_tap_queues, int, S_IRUGO);
> >>>>>> Please do not introduce new module paramaters. Please other ways to
> >>>>>> interchange values with userspace.
> >>>> I suggested this initially, but thinking more about it, I agree.
> >>>>
> >>>> It's a global limit (necessary to limit memory utilization by
> >>>> userspace), but it should be possible to change it
> >>>> after module load.
> >>> How about pass this limit through ifr during TUNSETIFF, then
> >>> alloc_netdev_mq() can use this limit.
> >> Any other ideas/comments from the experts. Or shall I re-repost other
> >> patches
> >> in the series except this patch until we agree on one.
> >>
> > It's kind of useless without a way for userspace to discover
> > how many queues it can create, no?
> >
> 
> We can implement ethtool_get_channels for tuntap. But I'm still not
> clear why this is necessary.

ethtool_get_channels for tuntap sounds good idea to retrieve number of queues
configured.
>