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Date: Mon, 16 Dec 2002 22:43:30 -0800 (PST)
From: dean gaudet <dean-list-linux-kernel@arctic.org>
To: Linus Torvalds <torvalds@transmeta.com>
cc: Dave Jones <davej@codemonkey.org.uk>, Ingo Molnar <mingo@elte.hu>,
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Subject: Re: Intel P6 vs P7 system call performance
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On Mon, 16 Dec 2002, Linus Torvalds wrote:

> It's not as good as a pure user-mode solution using tsc could be, but
> we've seen the kinds of complexities that has with multi-CPU systems, and
> they are so painful that I suspect the sysenter approach is a lot more
> palatable even if it doesn't allow for the absolute best theoretical
> numbers.

don't many of the multi-CPU problems with tsc go away because you've got a
per-cpu physical page for the vsyscall?

i.e. per-cpu tsc epoch and scaling can be set on that page.

the only trouble i know of is what happens when an interrupt occurs and
the task is rescheduled on another cpu... in theory you could test %eip
against 0xfffffxxx and "rollback" (or complete) any incomplete
gettimeofday call prior to saving a task's state.  but i bet that test is
undesirable on all interrupt paths.

-dean

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