svn commit: r227778 - head/sys/net

[Garrett Cooper]

On Sun, Nov 20, 2011 at 10:18 PM, Julien Ridoux
<jrid at cubinlab.ee.unimelb.edu.au> wrote:
>
> 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?

    struct inaddr, etc withstood sizing restrictions by adjusting the
sin_family / sin_len values appropriate to how large the payload was
made.. Could something similar be done for the ffcounter work (obscure
behind a void* pointer, use a proper bitwise ORed value for the
sin_len and sin_family, etc)?
Thanks,
-Garrett