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

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Date:   Tue, 31 Jan 2023 15:53:39 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Joerg Roedel <joro@...tes.org>
Cc:     "H. Peter Anvin" <hpa@...or.com>,
        Alexey Kardashevskiy <aik@....com>,
        Peter Zijlstra <peterz@...radead.org>, kvm@...r.kernel.org,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Jiri Kosina <jkosina@...e.cz>, Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [Question PATCH kernel] x86/amd/sev/nmi+vc: Fix stack handling
 (why is this happening?)

On Tue, Jan 31, 2023, Joerg Roedel wrote:
> 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!
> 
> The reason is that on SEV-ES only accesses to DR7 will cause #VC
> exceptions, DR0-DR6 are not intercepted.

I don't think that is technically true.  A _well-behaved_ hypervisor will not
intercept DR0-DR6 accesses for SEV-ES guests, but AFAICT nothing in the SEV-ES
architecture enforces that behavior.