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Subject: Re: [PATCH Part1 v5 38/38] virt: sevguest: Add support to get
 extended report
To:     Brijesh Singh <brijesh.singh@amd.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        linux-efi@vger.kernel.org, platform-driver-x86@vger.kernel.org,
        linux-coco@lists.linux.dev, linux-mm@kvack.org
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Joerg Roedel <jroedel@suse.de>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Ard Biesheuvel <ardb@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Sergio Lopez <slp@redhat.com>, Peter Gonda <pgonda@google.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
        David Rientjes <rientjes@google.com>,
        Tobin Feldman-Fitzthum <tobin@ibm.com>,
        Borislav Petkov <bp@alien8.de>,
        Michael Roth <michael.roth@amd.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        "Kirill A . Shutemov" <kirill@shutemov.name>,
        Andi Kleen <ak@linux.intel.com>, tony.luck@intel.com,
        marcorr@google.com, sathyanarayanan.kuppuswamy@linux.intel.com,
        Dov Murik <dovmurik@linux.ibm.com>
References: <20210820151933.22401-1-brijesh.singh@amd.com>
 <20210820151933.22401-39-brijesh.singh@amd.com>
 <bd5b2d47-2f66-87d9-ce1e-dfe717741d22@linux.ibm.com>
 <e7e35408-9336-2b89-6028-c201b406f5f3@amd.com>
From:   Dov Murik <dovmurik@linux.ibm.com>
Message-ID: <a49bafa9-e00c-7dec-ec07-08f4dfd6400f@linux.ibm.com>
Date:   Wed, 1 Sep 2021 11:32:08 +0300
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On 01/09/2021 0:11, Brijesh Singh wrote:
> 
> 
> On 8/31/21 3:22 PM, Dov Murik wrote:
>> Hi Brijesh,
>>
>> On 20/08/2021 18:19, Brijesh Singh wrote:
>>> Version 2 of GHCB specification defines NAE to get the extended guest
>>> request. It is similar to the SNP_GET_REPORT ioctl. The main difference
>>> is related to the additional data that be returned. The additional
>>> data returned is a certificate blob that can be used by the SNP guest
>>> user.
>>
>> It seems like the SNP_GET_EXT_REPORT ioctl does everything that the
>> SNP_GET_REPORT ioctl does, and more.  Why expose SNP_GET_REPORT to
>> userspace at all?
>>
>>
> 
> Since both of these options are provided by the GHCB protocol so I
> exposed it. Its possible that some applications may not care about the
> extended certificate blob. And in those case, if the hypervisor is
> programmed with the extended certificate blob and caller does not supply
> the enough number of pages to copy the blob then command should fail.
> This will enforce a new requirement on that guest application to
> allocate an extra memory. e.g:
> 
> 1. Hypervisor is programmed with a system wide certificate blob using
> the SNP_SET_EXT_CONFIG ioctl().
> 
> 2. Guest wants to get the report but does not care about the certificate
> blob.
> 
> 3. Guest issues a extended guest report with the npages = 0. The command
> will fail with invalid length and number of pages will be returned in
> the response.
> 
> 4. Guest will not need to allocate memory to hold the certificate and
> reissue the command.
> 
> The #4 is unnecessary for a guest which does not want to get. In this
> case, a guest can simply call the attestation report without asking for
> certificate blob. Please see the GHCB spec for more details.
> 

OK.  Originally I thought that by passing certs_address=NULL and
certs_len=0 the user program can say "I don't want this extra data"; but
now I understand that this will return an error (invalid length) with
number of pages needed.

-Dov