Re: [Question PATCH kernel] x86/amd/sev/nmi+vc: Fix stack handling (why is this happening?)

[Borislav Petkov]

On Mon, Jan 30, 2023 at 09:30:38AM -0800, H. Peter Anvin wrote:
> It's somewhat odd to me that reading %dr7 is volatile, but %dr6 is
> not... %dr6 is the status register!

Yeah, as a precaution I think we should make all those volatile. Just in
case.
 
> I believe they should all be volatile (the compiler semantics is that
> volatile operations are always executed exactly once, in strict
> program order with respect to any other volatile operations); the real
> question is if there should also be memory clobbers on %dr6 reads and
> any %dr write.

Yes, I think so too. From gcc docs:


"6.47.2.1 Volatile
.................

...

Note that the compiler can move even 'volatile asm' instructions
relative to other code, including across jump instructions."

We already have __FORCE_ORDER for exactly things like that.

-- 
Regards/Gruss,
    Boris.

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