linuxolator: tls_test results amd64

[Tijl Coosemans]

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.
> >
> > You mean:
> >
> > *kernel sets gsbase and switches back to user mode
> > *user program does things
> 
> Yes.  BTW, glibc seems to use movw instead of movl to load %gs.  I
> don't know if that makes difference, though.  It may have some
> effect when glibc is built with -mtls-direct-seg-refs flag.  Need
> confirmation.

I think there's no difference between movw and movl. The first stores
a 16 bit value in %gs, the second stores the lower 16 bits of a 32 bit
value.

That flag is interesting, but I think it only relates to using %gs
after it has been set up for tls.

> > *back in kernel mode, save gsbase into pcb and it appears to be 0
> > now?
> 
> 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?