Messages in this thread | | | Date | Thu, 18 Jun 2015 12:11:10 +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:
> > 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.)
The most deadly complexity in our asm code are IMHO the intertwined threads of control flow - all of that should go into C, where it's much easier to see what's going on, because we have named variables, established code patterns and a compiler checking for common mistakes and such.
The other big area of complexity are our partial save/restore tricks, which makes tracking of what is saved (and what is not) tricky and fragile.
Thanks,
Ingo
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