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

From: Ingo Molnar
Date: Thu Jun 18 2015 - 06:11:25 EST

* Andy Lutomirski <luto@xxxxxxxxxxxxxx> 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.


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