Thread Local Storage

Peter Wemm peter at wemm.org
Mon Mar 29 15:18:34 PST 2004


On Monday 29 March 2004 02:36 pm, Daniel Eischen wrote:
[..]
> > > > You don't need a syscall at thread switch if you do something
> > > > like:
> > > >
> > > > 	_thread_switch(...)
> > > > 	{
> > > > 		if (tcb doesn't have LDT entry) {
> > > > 			if (!free LDT entries)
> > > > 				steal LDT entry from non-running thread;
> > > > 			allocate LDT entry and point it at TLS goop for tcb.
> > > > 		}
> > > > 		load_gs(tcb's LDT sel);
> > >
> > > That's a system call on amd64.
> >
> > I'm not quite up to speed on amd64. So in 64-bit mode it doesn't
> > really have an LDT at all, is that right?
>
> I'm not sure, but you have to make a system call to set it
> or it's equivalent (amd64_set_fsbase()).

Correct.  There are two ways to do these things on this cpu.  One is to 
use descriptor tables.  The catch is that using descriptor tables 
forces a 4GB limit.  It won't wrap around.  The other way is to write 
to the MSRs for fsbase/gsbase.  But the downside of that is that is a 
priviliged operation and needs to be done in supervisor mode.

I don't *have* an LDT on the amd64 kernel.  I'm dreading having to 
emulate the i386 sysarch LDT stuff already.
-- 
Peter Wemm - peter at wemm.org; peter at FreeBSD.org; peter at yahoo-inc.com
"All of this is for nothing if we don't go to the stars" - JMS/B5


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