Messages in this thread | | | From | Andy Lutomirski <> | Date | Thu, 18 Jun 2015 04:06:22 -0700 | Subject | Re: [RFC/INCOMPLETE 00/13] x86: Rewrite exit-to-userspace code |
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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.
--Andy
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