Re: [RFC/INCOMPLETE 00/13] x86: Rewrite exit-to-userspace code

[Ingo Molnar <mingo@kernel.org>]

* Andy Lutomirski <luto@amacapital.net> wrote:

> On Thu, Jun 18, 2015 at 3:11 AM, Ingo Molnar <mingo@kernel.org> wrote:
> >
> > * Andy Lutomirski <luto@amacapital.net> wrote:
> >
> >> > The only low level bits remaining in assembly will be low level hardware ABI
> >> > details: saving registers and restoring registers to the expected format - no
> >> > 'active' code whatsoever.
> >>
> >> I think this is true for syscalls.  Getting the weird special cases (IRET and 
> >> GS fault) for error_entry to work correctly in C could be tricky.
> >
> > Correct, and I double checked the IRET fault path yesterday (fixup_bad_iret), 
> > and it looks like a straightforward exception handler with limited control 
> > flow. It can stay in asm just fine, it seems mostly orthogonal to the rest.
> >
> > I didn't check the GS fault path, but that only affects 32-bit, as we use 
> > SWAPGS on 64-bit, right? In any case, that code too (32-bit RESTORE_REGS) 
> > belongs into the natural 'hardware ABI preparation code' that should stay in 
> > assembly. (Unless I missed some other code that might cause trouble.)
> 
> Look for "gs_change".  To change the gs selector, we do swapgs, then load gs, 
> then swapgs again.  If the gs load fails, then we trigger a special fixup.

Yes, but I don't see the connection to moving the syscall (and IRQ) entry code to 
.c: native_load_gs_index() is a separate API we call from regular kernel code, and 
it has a regular exception fixup entry plus a trap handler special case.

It's fine in entry_64.S, but it would be equally fine in inline asm() as well.

I think it's fine in entry_64.S as long as the error trap code (which refers to 
the change_gs RIP) lives there. But it could live in .c as well, as we can 
generate global symbols within inline asm() too.

Thanks,

	Ingo