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

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Message-ID: <Y9eLLGug11f+kOb+@8bytes.org>
Date:   Mon, 30 Jan 2023 10:17:32 +0100
From:   Joerg Roedel <joro@...tes.org>
To:     Alexey Kardashevskiy <aik@....com>
Cc:     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>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [Question PATCH kernel] x86/amd/sev/nmi+vc: Fix stack handling
 (why is this happening?)

On Sat, Jan 28, 2023 at 02:52:23PM +0100, Joerg Roedel wrote:
> Yeah, something like this will be the fix. I am still thinking about
> the right place to put the volatile to make it explicit to the situation
> we are encountering here (which is SEV-ES specific).
> 
> Best would be an explicit barrier in C code between sev_es_ist_enter()
> and the DR7 read, but all barriers I tried to far only seem to affect
> memory instructions and had no influence on the DR7 read (which is
> obviously not considered as a memory read by the compiler).
> 
> The best place to put the barrier is in the sev_es_ist_enter() inline
> function, right after the static_call to __sev_es_ist_enter().

Okay, after some investigation I was not able to find a compiler barrier
which affects DR7 read ordering. This leaves us with the only solution
of directly forbidding DR7 register access re-ordering by adding a
volatile to the asm, like you did before.

I will send a fix later today.

Regards,

	Joerg