Date: Tue, 30 Mar 2004 09:25:02 +0100 From: Doug Rabson <dfr@nlsystems.com> To: Peter Wemm <peter@wemm.org> Cc: freebsd-threads@freebsd.org Subject: Re: Thread Local Storage Message-ID: <200403300925.02485.dfr@nlsystems.com> In-Reply-To: <200403291518.33559.peter@wemm.org> References: <Pine.GSO.4.10.10403291718320.29683-100000@pcnet5.pcnet.com> <200403291518.33559.peter@wemm.org>
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On Tuesday 30 March 2004 00:18, Peter Wemm wrote:
> 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.
What do you put in %fs and %gs for the non-table mode?
The TLS ABI for amd64 is a 64bit equivalent to the GNU i386 ABI (with
%fs instead of %gs). It also allows stuff like 'movq %fs:x@tpoff, %rax'
where x@tpoff resolves to the negative offset from the end of the TLS
block to the location of x. I can't see any way of avoiding setting
fsbase on thread switch here.
>
> I don't *have* an LDT on the amd64 kernel. I'm dreading having to
> emulate the i386 sysarch LDT stuff already.
Segment registers. Not the worlds greatest idea...
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