linux-kernel - Re: [PATCH v3] x86/sev: Add SEV-SNP guest feature negotiation support

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Message-ID: <05171aad-bcef-a731-970f-726b393e3ad3@google.com>
Date:   Tue, 3 Jan 2023 15:25:36 -0800 (PST)
From:   David Rientjes <rientjes@...gle.com>
To:     Borislav Petkov <bp@...en8.de>
cc:     "Nikunj A. Dadhania" <nikunj@....com>,
        linux-kernel@...r.kernel.org, x86@...nel.org, kvm@...r.kernel.org,
        mingo@...hat.com, tglx@...utronix.de, dave.hansen@...ux.intel.com,
        seanjc@...gle.com, pbonzini@...hat.com, thomas.lendacky@....com,
        michael.roth@....com, stable@...nel.org
Subject: Re: [PATCH v3] x86/sev: Add SEV-SNP guest feature negotiation
 support

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.