Messages in this thread | | | Date | Thu, 18 Jun 2015 18:24:22 +0200 | From | Ingo Molnar <> | Subject | Re: [RFC/INCOMPLETE 00/13] x86: Rewrite exit-to-userspace code |
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* 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
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