lkml.org 
[lkml]   [2015]   [Jun]   [18]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [RFC/INCOMPLETE 00/13] x86: Rewrite exit-to-userspace code

* 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


\
 
 \ /
  Last update: 2015-06-18 12:21    [W:0.524 / U:0.880 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site