patch for %gs saving

[Julian Elischer]

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.



> 
> -- 
> Dan Eischen
> 
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