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Date: Fri, 11 Apr 2003 02:02:06 -0400 (EDT)
From: Daniel Eischen <eischen@pcnet1.pcnet.com>
To: Peter Wemm <peter@wemm.org>
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cc: David Xu <davidxu@freebsd.org>
cc: freebsd-threads@freebsd.org
Subject: Re: patch for %gs saving 
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On Thu, 10 Apr 2003, Peter Wemm wrote:

> "David Xu" wrote:
> > Here is the patch for kernel to save %gs,
> > it works well on my machine.
> > http://people.freebsd.org/~davidxu/i386_gs.diff
> > Daniel, is this the reason in your libpthread
> > patch that doesn't use getcontext syscall ?
> 
> To put some background on the issue, there is a reason why we did not
> do this.  %gs is not used by the kernel, so it does not normally need to
> be saved and restored on every trap into the kernel.  Setting a segment
> register is Really Slow - measured in hundreds of clock cycles.
> 
> So, we normally only touch %gs when we context switch to a different process
> that may have a different %gs.  Or when one of the context syscalls wants
> it changed.  We cannot avoid touching %fs because we use it for kernel
> private data.  But if it wasn't for that, we wouldn't be touching
> %fs for regular traps/syscalls/etc either.
> 
> Bruce Evans understands this better than I do, I would suggest not making
> this change without talking about it with him first.

BTW, it's not really a big deal for the UTS to restore %gs
(or probably whatever it ends up being on other archs) before
continuing a thread.

-- 
Dan Eischen