patch for %gs saving

[David Xu]

----- Original Message ----- 
From: "Julian Elischer" <julian at elischer.org>
To: "Daniel Eischen" <eischen at pcnet1.pcnet.com>
Cc: "Peter Wemm" <peter at wemm.org>; "David Xu" <davidxu at freebsd.org>; <freebsd-threads at freebsd.org>
Sent: Friday, April 11, 2003 2:54 PM
Subject: Re: patch for %gs saving 


> 
> 
> On Fri, 11 Apr 2003, Daniel Eischen wrote:
> 
> > 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.
> 
> I put it to you that %gs should be a way of finding the current KSE
> mailbox
> (upcall mailbox) which will NOT CHANGE when a thread is changed in
> userland. one simply changes the curthread pointer in the KSE (upcall)
> mailbox, leaving %gs the same.
> 
> %gs should never change for a single KSE as it goes in and out of the
> kernel, it must always have the same mailbox.
> 
> 

Yes, I saw some code in libpthread patch changes %gs, for
example thr_switch(), thr_enter_uts(), I think such code 
needn't change %gs, and I think even %fs, %ds, %es is needn't
to be changed, this would speedup context switch.

> 
> > 
> > -- 
> > Dan Eischen
> > 
> > _______________________________________________
> > freebsd-threads at freebsd.org mailing list
> > http://lists.freebsd.org/mailman/listinfo/freebsd-threads
> > To unsubscribe, send any mail to "freebsd-threads-unsubscribe at freebsd.org"
> >