Re: [patch] enabling APIC and NMI watchdog on UP systems

[Jamie Lokier]

Philip J. Mucci wrote:
> Basically, reading the counters is a 2 step process: read the mmapped
> virtual counters and add that to the contents of rdpmc(). This means
> that (at least for the x86 series) the kernel interface only needs to
> 'guard' access to the MSR to make sure the user doesn't set up anything
> pathological. This same kind of interface is also possible for the
> UltraSparc. For other systems where a lower IPL is required, the
> interface needs to be a fast-path syscall interface.

That sounds similar to the vsyscall interface Ingo's been working on.
Why not use the read-only kernel page used for fast gettimeofday()
calibration with MSR offsets?

It's slightly more overhead than inlining the code, but it is still only
a small overhead (no kernel entry, just a function call to code in a
user-readable page).  People doing advanced dynamic compilation
techniques can always trace the vsyscall code and inline equivalent code
directly into their hot paths ;-)

-- Jamie