Re: [PATCH] arch: x86: power: cpu: init %gs before __restore_processor_state (clang)

[Roman Kiryanov]

On Fri, Sep 18, 2020 at 3:25 PM Pavel Machek <pavel@xxxxxxx> wrote:
>
> On Tue 2020-09-15 11:36:13, Roman Kiryanov wrote:
> > On Tue, Sep 15, 2020 at 11:27 AM Borislav Petkov <bp@xxxxxxxxx> wrote:
> > > > I believe the kernel makes a questionable assumption on how clang
> > > > uses registers (gs will not be used if stack protection is disabled).
> > > > Both kernel and clang behaves unfortunate here.
> > >
> > > If the kernel is at fault here and this same thing happens with GCC,
> > > sure, but this is a clang-specific fix.
> >
> > This is fair. Unfortunately I am not an x86 asm expert. I expect the proper
> > fix should land into arch/x86/kernel/acpi/wakeup_64.S to init %gs
> > (maybe some more registers) before "jmp restore_processor_state".
>
> That would certainly be nicer / more acceptable solution than patch
> being proposed here.
>
> Code was written with assumption compiler random C code would not use
> %gs. If that's no longer true, fixing it in wakeup_64.S _with comments
> explaining what goes on_ might be solution.

I looked and restore_processor_state is referenced in several places,
so changing
wakeup_64.S is not enough. Is moving the beginning of restore_processor_state
to an .S file ok? I see restore_processor_state initializes CR registers first,
do you know if there is a reason to do so (can I init segment
registers before CR ones)?

Regards,
Roman.