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Subject: Re: [RFC][PATCH] linux-2.5.34_vsyscall_A0
From: john stultz <johnstul@us.ibm.com>
To: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Stephen Hemminger <shemminger@osdl.org>, Andrea Arcangeli <andrea@suse.de>,
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On Mon, 2002-10-21 at 06:18, Alan Cox wrote:
> On Fri, 2002-10-18 at 17:13, Stephen Hemminger wrote:
> > It would be great to rework the whole TSC time of day stuff to work with
> > per cpu data and allow unsychronized TSC's like NUMA. The problem is
> > that for fast user level access, there would need to be some way to find
> 
> The timer isnt even necessarily constant rate. The tsc is a nice tool
> for debugging. Using it as a clock was not in the long run brilliant.
> Don't try and continue it further, we have ACPI and HPET and other
> better solutions in upcoming PC hardware.

Yes, I also feel all this per-cpu TSC stuff is not the way to go (on top
of all this per-cpu mapping, etc. you'd also have to round-robin the
timer interrupt so each cpu has a last_tsc_low value, then if cpus are
varying in freq you have to recalc that occasionally. it just gets
messy.). The current vsyscall implementation uses the TSC because on 99%
of the boxes out there, the TSC is in sync and works fine as time source
(ie: normal gettimeofday uses it). On boxes that don't use the TSC, the
vsyscall shouldn't be mapped in until we have an alternate HPET(equv)
vsyscall solution. 

I'll see if I can clean that up later today. I also didn't mind Andrea's
suggestion for using a /proc entry to disable/check for vsyscalls, so I
might give that a whirl as well. 

thanks for all the feedback, everyone!
-john

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