svn commit: r227778 - head/sys/net

[Julien Ridoux]

On 21/11/2011, at 4:39 PM, Lawrence Stewart wrote:

> On 11/21/11 16:12, Ben Kaduk wrote:
>> On Sun, Nov 20, 2011 at 11:17 PM, Lawrence Stewart<lstewart at freebsd.org>  wrote:
>>> Author: lstewart
>>> Date: Mon Nov 21 04:17:24 2011
>>> New Revision: 227778
>>> URL: http://svn.freebsd.org/changeset/base/227778
>>> 
>>> Log:
>>>  - When feed-forward clock support is compiled in, change the BPF header to
>>>    contain both a regular timestamp obtained from the system clock and the
>>>    current feed-forward ffcounter value. This enables new possibilities including
>> 
>> Is it really necessary to make the ABI dependent on a kernel
>> configuration option?  This causes all sorts of headaches if loadable
>> modules ever want to use that ABI, something that we just ran into
>> with vm_page_t and friends and had a long thread on -current about.
> 
> Fair question. Julien, if pcap and other consumers will happily ignore the new ffcount_stamp member in the bpf header, is there any reason to conditionally add the ffcounter into the header struct?

It is a valid question indeed. The feedback I have received so far was to not have the feed-forward clock support be a default kernel configuration option. What follows is based on this assumption.

The commit (r227747) introduces sysctl that are conditioned by the same "FFCLOCK" kernel configuration option. If a loadable module tests for the presence of this sysctl, it will know if the ffcount_stamp member is available. Is it too much of a hack?

Alternatively, if the ffcounter is added to the bpf header unconditionally, the ffcount_stamp member can be set to 0. Loadable modules will then see a consistent ABI but will retrieve a meaningless value.

I am not sure which option makes more sense, any preference?

Cheers,
Julien