Date: Mon, 29 Mar 2004 23:16:17 +0100 From: Doug Rabson <dfr@nlsystems.com> To: Daniel Eischen <eischen@vigrid.com> Cc: freebsd-threads@freebsd.org Subject: Re: Thread Local Storage Message-ID: <200403292316.17288.dfr@nlsystems.com> In-Reply-To: <Pine.GSO.4.10.10403291651380.23090-100000@pcnet5.pcnet.com> References: <Pine.GSO.4.10.10403291651380.23090-100000@pcnet5.pcnet.com>
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On Monday 29 March 2004 22:56, Daniel Eischen wrote:
> On Mon, 29 Mar 2004, Doug Rabson wrote:
> > On Monday 29 March 2004 22:26, Daniel Eischen wrote:
> > > On Mon, 29 Mar 2004, Doug Rabson wrote:
> > > > Surely the GNU TLS ABI is preferable? It generates much smaller
> > > > code and needs many fewer relocations.
> > >
> > > No, we don't want an LDT for every thread and don't want
> > > to force a syscall for a thread switch.
> >
> > But the code it generates is at least twice the size for dynamic
> > TLS. It seems that the GNU people have done a better job defining
> > the TLS abi for i386.
>
> About the only thing that uses TLS that I know is nvidia's
> openGL. If you design an API correctly, there's no need
> for TLS. I would hope that it's usage would be limited.
I'd quite like to see us use it for stuff like errno, _res and other
uglification currently in libc. Not until the 6.x timeframe though.
>
> > 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?
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