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Date:   Mon, 2 Jan 2023 20:42:11 +0100
From:   Borislav Petkov <bp@alien8.de>
To:     "Nikunj A. Dadhania" <nikunj@amd.com>
Cc:     David Rientjes <rientjes@google.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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On Mon, Jan 02, 2023 at 08:50:23PM +0530, Nikunj A. Dadhania wrote:
> I think the "why" part depends on the user. Whether or not the user needs a
> certain feature enabled for the confidential guest.
> 
> If the cloud provider(hypervisor) enables the feature on user request, the
> guest terminates with GHCB_SNP_FEAT_NOT_IMPLEMENTED when guest kernel does
> have corresponding code/implementation.

I think you mean "does not have" here.

In any case, I think this whole handling of SEV features could go both ways:

* Cloud provider could say: we've enabled features X, Y and Z and if the guest
doesn't have support for them, then it would fail booting.

There would optimally be some text sowewhere in the cloud provider documentation
stating why those features are enabled and thus required to be supported by the
guest.

* Guest owner could require a minimal subset of features which must be present
in the HV in order to even boot on that HV.

Of course, I'm only speculating here. How it ends up really playing out in
reality we will have to see...

-- 
Regards/Gruss,
    Boris.

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