linuxolator: tls_test results amd64

[Jung-uk Kim]

On Tuesday 23 January 2007 06:12 pm, Jung-uk Kim wrote:
> On Tuesday 23 January 2007 05:59 pm, Tijl Coosemans wrote:
> > 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.
>
> That's correct.  See:
>
> http://sourceware.org/cgi-bin/cvsweb.cgi/~checkout~/linuxthreads/li
>nuxthreads/sysdeps/i386/tls.h?rev=1.39&content-type=text/plain&cvsro
>ot=glibc

I have read through entire segment register section of AMD64 manual 
and I understand the whole picture now.  fsbase/gsbase is not 
restored for 32-bit thread because it is ignored in Compatibility 
Mode.  In Compatibility Mode, the segment regiter load is exactly 
like legacy x86 mode, i.e., hidden portion is not loaded from 
fsbase/gsbase but from GDT.  So, you're right; there is no other way 
to implement this correctly without setting up segment 
descriptor. :-(

Jung-uk Kim