linux-kernel - Re: [PATCH Part1 RFC v3 11/22] x86/sev: Add helper for validating pages in early enc attribute changes

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Date:   Wed, 16 Jun 2021 08:10:22 -0500
From:   Brijesh Singh <brijesh.singh@....com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     brijesh.singh@....com, x86@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-efi@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        linux-crypto@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>, Peter Gonda <pgonda@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>, tony.luck@...el.com,
        npmccallum@...hat.com
Subject: Re: [PATCH Part1 RFC v3 11/22] x86/sev: Add helper for validating
 pages in early enc attribute changes


On 6/16/21 8:02 AM, Borislav Petkov wrote:
> On Wed, Jun 16, 2021 at 07:49:25AM -0500, Brijesh Singh wrote:
>> If you still think ...
> I think you should answer my question first:
>
>> Imagine you're a guest owner and you haven't written the SNP code and
>> you don't know how it works.
>>
>> You start a guest in the public cloud and it fails because the
>> hypervisor violates the GHCB protocol and all that guest prints before
>> it dies is
>>
>> "general request termination"
>>
>> How are you - the guest owner - going to find out what exactly happened?
>>
>> Call support?
> And let me paraphrase it again: if the error condition with which the
> guest terminates is not uniquely identifiable but simply a "general
> request", how are such conditions going to be debugged?

I thought I said it somewhere in our previous conversation, I would look
at the KVM trace log, each vmgexit entry and exit are logged. The log
contains full GHCB MSR value, and in it you can see both the request and
response code and decode the failure reason.

-Brijesh