linux-kernel - Re: [RFC 2/4] tuntap: Publish tuntap maximum number of queues as module

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Date:	Wed, 20 Aug 2014 13:53:44 +0200
From:	Jiri Pirko <jiri@...nulli.us>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Pankaj Gupta <pagupta@...hat.com>, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, davem@...emloft.net, jasowang@...hat.com,
	dgibson@...hat.com, vfalico@...il.com, edumazet@...gle.com,
	vyasevic@...hat.com, hkchu@...gle.com, wuzhy@...ux.vnet.ibm.com,
	xemul@...allels.com, therbert@...gle.com,
	bhutchings@...arflare.com, xii@...gle.com,
	stephen@...workplumber.org
Subject: Re: [RFC 2/4] tuntap: Publish tuntap maximum number of queues as
 module_param

Wed, Aug 20, 2014 at 01:49:07PM CEST, mst@...hat.com wrote:
>On Wed, Aug 20, 2014 at 01:46:20PM +0200, Jiri Pirko wrote:
>> Wed, Aug 20, 2014 at 01:17:24PM CEST, mst@...hat.com wrote:
>> >On Wed, Aug 20, 2014 at 12:58:17PM +0200, Jiri Pirko wrote:
>> >> Mon, Aug 18, 2014 at 03:37:18PM CEST, pagupta@...hat.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@...hat.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.
>> >
>> >Additionally, userspace that has the FD should be able to
>> >retrieve the value without guessing that the FD is
>> >for the tun device (and not e.g. macvtap).
>> >To retrieve the value, an ioctl is probably the
>> >cleanest approach.
>> >
>> >To set it, how about a sysctl? I think the limit can also apply to
>> >all devices, not just tun.
>> 
>> Or netlink?
>
>Are there examples of netlink being used to set global defaults
>as opposed to per-device parameters?

That is so far not possible. But I believe that it can be implemented.
I'm just thinking out loud.

>
>> >
>> >-- 
>> >MST
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