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Date:   Tue, 3 Jan 2023 15:25:36 -0800 (PST)
From:   David Rientjes <rientjes@google.com>
To:     Borislav Petkov <bp@alien8.de>
cc:     "Nikunj A. Dadhania" <nikunj@amd.com>,
        linux-kernel@vger.kernel.org, x86@kernel.org, kvm@vger.kernel.org,
        mingo@redhat.com, tglx@linutronix.de, dave.hansen@linux.intel.com,
        seanjc@google.com, pbonzini@redhat.com, thomas.lendacky@amd.com,
        michael.roth@amd.com, stable@kernel.org
Subject: Re: [PATCH v3] x86/sev: Add SEV-SNP guest feature negotiation
 support
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References: <20230102083810.71178-1-nikunj@amd.com> <3169b54b-d990-7707-5ec4-cde7261318fe@google.com> <45487a87-764a-7ff3-292b-4a55fe29f7ba@amd.com> <Y7M4IzNYBtfEJe6Z@zn.tnic> <02a2e641-6b6d-e1c5-2c43-8815306d92a0@amd.com> <Y7QRkzxg1HW9wMCp@zn.tnic>
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On Tue, 3 Jan 2023, Borislav Petkov wrote:

> On Tue, Jan 03, 2023 at 09:07:14AM +0530, Nikunj A. Dadhania wrote:
> > Currently, GHCBData[24:63] is unused. If we intend to use the bit range(40bits), GHCB spec 
> > will need to be updated. And probably would not be enough.
> 
> My fear too...
> 
> > As the termination request is done using GHCB MSR protocol, exit codes cannot be used.
> 
> We need to figure out some other way of communicating to the guest owner because
> of which feature the guest refused booting.
> 

Yeah, this was exactly the thought.  If the guest terminates because 
SNP_FEAT_NOT_IMPLEMENTED, it seems necessary to provide information on 
which feature(s) the guest is lacking an implementation for that the 
hypervisor has enabled.  Otherwise this becomes a frustrating trial and 
error for the customer.