IFCAP_CANTCHANGE ? (Re: RFC: need one IFCAP bit for NETMAP)

On Tue, Jun 07, 2011 at 08:14:17AM +0200, Luigi Rizzo wrote:
> hi,
> in order to import netmap support into the tree (see
> http://info.iet.unipi.it/~luigi/netmap ), i need
> to use one of the if_cap{abilities|enable} bits to report
> that the interface supports this mode and record whether
> the mode is enabled or not.
> 
> Bit  0x100000 is currently the first unused bit so if
> there are no objections i will add the following
> to sys/net/if.h
> 
> #define IFCAP_NETMAP    0x100000 /* netmap mode supported/enabled */
> 
> and the following change to sbin/ifconfig/ifconfig.c
> 
> -"\21VLAN_HWFILTER\23VLAN_HWTSO\24LINKSTATE"
> +"\21VLAN_HWFILTER\23VLAN_HWTSO\24LINKSTATE\25NETMAP"
> 
> That's all you need, as netmap mode is not supposed to be
> set from sbin/ifconfig.

speaking of which, it occurs to me that while we have a
"CANTCHANGE" category for interface flags, we don't have
one for capabilities. Would it be ok to add one ?
Looking at the list in if.h, for example, it seems to me
that things like IFCAP_LINKSTATE or IFCAP_VLAN_MTU or IFCAP_JUMBO_MTU
shouldn't be modifiable through SIOCSIFCAP.

Of course if would be best to separate CANTCHANGE bits from
those that can be modified through ioctls, but that would
apply to interface flags as well, and i don't think it can be
done due to API change issues.

cheers
luigi