Date: Wed, 23 Nov 2011 01:00:57 +1100 From: Lawrence Stewart <lstewart@freebsd.org> To: John Baldwin <jhb@FreeBSD.org> Cc: svn-src-head@FreeBSD.org, svn-src-all@FreeBSD.org, src-committers@FreeBSD.org, Julien Ridoux <jrid@cubinlab.ee.unimelb.edu.au>, Ben Kaduk <minimarmot@gmail.com> Subject: Re: svn commit: r227778 - head/sys/net Message-ID: <4ECBAB19.4010907@freebsd.org> In-Reply-To: <201111220830.05029.jhb@freebsd.org> References: <201111210417.pAL4HOdi023556@svn.freebsd.org> <648D11A8-3636-49E5-BF20-83E4EA87242C@cubinlab.ee.unimelb.edu.au> <4EC9FD8A.5040401@freebsd.org> <201111220830.05029.jhb@freebsd.org>
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On 11/23/11 00:30, John Baldwin wrote: > On Monday, November 21, 2011 2:28:10 am Lawrence Stewart wrote: >> On 11/21/11 17:18, Julien Ridoux 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@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? >> >> If I understand the issues correctly, I think the appropriate path >> forward is to remove the conditional change to the bpf header and have >> ffcount_stamp become a permanent member of the struct. We'll just leave >> the member uninitialised in the !FFCLOCK case. This change will make the >> patch un-MFCable, but I think that's ok. >> >> As to the issue of how a kernel module would detect if it's being loaded >> into a FFCLOCK enabled kernel, why wouldn't we expect modules to >> "#include opt_ffclock.h" and conditionally compile code based on FFCLOCK >> being defined? Is there a use case for run-time (as opposed to >> compile-time) module detection of feed-forward clock capabilities? > > Think of standalone modules that are not built as part of a kernel (e.g. > 3rd party device drivers). In general we should avoid having structures > change size for kernel options, especially common structures. It just adds > lots of pain and suffering and complexity. We are stuck with it for PAE on > i386 (which causes pain), and for LOCK_PROFILING (but that is sufficiently > rare and expensive it seems to be ok). I think 8 bytes for bpf packet is > not sufficiently expensive to justify the extra headache. Just always leave > the new field in. hmm... Julien almost has a patch finished which accomplishes what my most recent email in this thread describes. Julien, I suggest we get it finished and follow up to this thread with a pointer to the patch for people to look at. If there's still a strong feeling that what it does is going to bring pain we can do away with the new BPF_FFCOUNTER config option and have the bpf header struct just grow by 8 bytes. Stay tuned... Cheers, Lawrence
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