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<lstew...@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?
Cheers,
Lawrence
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