patch for %gs saving

[Daniel Eischen]

On Fri, 11 Apr 2003, David Xu wrote:
> ----- Original Message ----- 
> From: "Daniel Eischen" <eischen at pcnet1.pcnet.com>
> To: "David Xu" <davidxu at freebsd.org>
> Cc: <freebsd-threads at freebsd.org>
> Sent: Friday, April 11, 2003 1:55 PM
> Subject: Re: patch for %gs saving
> 
> 
> > On Fri, 11 Apr 2003, 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
> > 
> > Thanks, I'll give it a go.
> > 
> > > Daniel, is this the reason in your libpthread
> > > patch that doesn't use getcontext syscall ?
> > 
> > No, we already had userland versions of getcontext()
> > so I simply reused them to avoid the system call.
> > That's why THR_GETCONTEXT is a macro; it can be
> > defined to be getcontext() for those archs that
> > don't have userland versions and want to use the
> > system call instead.
> > 
> > Note that we still need userland versions of
> > _thread_switch() and _thread_enter_uts() anyways,
> > so writing a userland [gs]etcontext() is probably
> > pretty simple once you have those.
> > 
> > Plus, when you get a context in order to make a
> > context for a new thread, you still can't rely on
> > %gs because it may be scheduled on another KSE or
> > the thread could be a scope system thread in which
> > case it be scheduled in another KSEG.  So the current
> > %gs isn't necessarily correct for a new thread.
> > 
> > Hmm, this raises a good point.  Once you set up a
> > thread to run a signal handler, the %gs register has
> > already been set.  We have to be sure that the
> > interrupted context and the thread's new (signal)
> > context both have the same %gs and that it runs on
> > the correct KSE.  Either that, or we have to be able
> > to change the contexts to be the correct %gs before
> > running the thread and invoking the handler.
> > 
> 
> I think those code shouldn't touch %gs, these includes
> _thread_switch(), _thread_enter_uts(), they don't know
> there is a %gs register. Execept kse_entry(), when first
> time an upcall is made, it sets %gs to its own LDT, I 
> think you have already done this, but _thread_switch(),
> and _thread_enter_uts() might be change to not touch %gs.

getcontext() and setcontext() still touch %gs though.
If an application uses those functions and they are
used in a thread once while it is on one KSE and
another time while it is on a different KSE, then
%gs gets hosed.

-- 
Dan Eischen