From owner-freebsd-emulation@FreeBSD.ORG  Tue Jan 23 22:59:11 2007
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From: Tijl Coosemans <tijl@ulyssis.org>
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Date: Tue, 23 Jan 2007 23:59:04 +0100
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Cc: Jung-uk Kim <jkim@freebsd.org>
Subject: Re: linuxolator: tls_test results amd64
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On Tuesday 23 January 2007 23:43, Tijl Coosemans wrote:
> On Tuesday 23 January 2007 21:55, Jung-uk Kim wrote:
> > On Tuesday 23 January 2007 03:13 pm, Tijl Coosemans wrote:
> > > On Tuesday 23 January 2007 20:00, Jung-uk Kim wrote:
> > > > Second problem is MSR_KGSBASE is scrubbed by something during
> > > > context switch, i.e., it becomes 0 some times.
> >
> > Saved pcb_gsbase seems always correct.  MSR_KGSBASE is not, which is 
> > supposedly swapped with MSR_GSBASE via swapgs.  Maybe I am confused, 
> > or maybe my CPU is too old (it's C0 stepping and I know there are 
> > some segmentation issues with the revision) but that's what I see. I 
> > need more time for testing (or resting?).
> 
> Ok, I understand why pcb_gsbase is always correct. It is never written
> to except for cpu_set_user_tls, i386_set_gsbase and set_thread_area.
> cpu_switch only reads from it to restore gsbase. At least, that's what
> cpu_switch does in case of 64 bit programs, not 32 bit it seems. Why
> is that? Why is there a jmp on line 186 in sys/amd64/amd64/cpu_switch.S?

This doesn't explain why gsbase becomes 0 by the way. I still think
that's because glibc does "mov index,%gs" after calling set_thread_area.