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

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Date:   Mon, 30 Jan 2023 19:04:50 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     "H. Peter Anvin" <hpa@...or.com>
Cc:     Alexey Kardashevskiy <aik@....com>, Joerg Roedel <joro@...tes.org>,
        Peter Zijlstra <peterz@...radead.org>, kvm@...r.kernel.org,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Sean Christopherson <seanjc@...gle.com>,
        Jiri Kosina <jkosina@...e.cz>, Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [Question PATCH kernel] x86/amd/sev/nmi+vc: Fix stack handling
 (why is this happening?)

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.

https://people.kernel.org/tglx/notes-about-netiquette