Date:   Tue, 31 Jan 2023 15:53:39 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Joerg Roedel <joro@8bytes.org>
Cc:     "H. Peter Anvin" <hpa@zytor.com>,
        Alexey Kardashevskiy <aik@amd.com>,
        Peter Zijlstra <peterz@infradead.org>, kvm@vger.kernel.org,
        x86@kernel.org, linux-kernel@vger.kernel.org,
        Thomas Gleixner <tglx@linutronix.de>,
        Jiri Kosina <jkosina@suse.cz>, Ingo Molnar <mingo@redhat.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Borislav Petkov <bp@alien8.de>,
        Tom Lendacky <thomas.lendacky@amd.com>
Subject: Re: [Question PATCH kernel] x86/amd/sev/nmi+vc: Fix stack handling
 (why is this happening?)
Message-ID: <Y9k5g5jYA/rjIwUj@google.com>
References: <20230127035616.508966-1-aik@amd.com>
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 <b7880f0b-a592-cf2d-03b9-1ccfd83f8223@amd.com>
 <Y9QI9JwCVvRmtbr+@8bytes.org>
 <3bb3e080-caee-8bc8-7de9-f44969f16e75@amd.com>
 <38C572D7-E637-48C2-A57A-E62D44FF19BB@zytor.com>
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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.