On Sat, Mar 26, 2011 at 10:12:32AM -0400, John Baldwin wrote: > On Saturday, March 26, 2011 08:16:46 am Peter Jeremy wrote: > > On 2011-Mar-25 08:18:38 -0400, John Baldwin <j...@freebsd.org> wrote: > > >For modern Intel CPUs you can just assume that the TSCs are in sync across > > >packages. They also have invariant TSC's meaning that the frequency > > >doesn't change. > > > > Synchronised P-state invariant TSCs vastly simplify the problem but > > not everyone has them. Should the fallback be more complexity to > > support per-CPU TSC counts and varying frequencies or a fallback to > > reading the time via a syscall? > > I think we should just fallback to a syscall in that case. We will also need > to do that if the TSC is not used as the timecounter (or always duplicate the > ntp_adjtime() work we do for the current timecounter for the TSC timecounter). > > Doing this easy case may give us the most bang for the buck, and it is also a > good first milestone. Once that is in place we can decide what the value is > in extending it to support harder variations. > > One thing we do need to think about is if the shared page should just export a > fixed set of global data, or if it should export routines. The latter > approach is more complex, but it makes the ABI boundary between userland and > the kernel more friendly to future changes. I believe Linux does the latter > approach? Linux uses a so-called vdso, which is linked into the process.
I think that the efforts to implement a vdso approximately equal to the efforts required to implement timecounters in the user mode. On the other hand, with vdso we could properly annotate signal trampolines with the unwind info, that is also a big win.
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