linux-kernel - Re: [PATCH v8 27/40] x86/boot: Add Confidential Computing type to setup

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Date:   Mon, 13 Dec 2021 08:49:41 -0600
From:   Brijesh Singh <brijesh.singh@....com>
To:     Dave Hansen <dave.hansen@...el.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
Cc:     brijesh.singh@....com, 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>,
        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>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Borislav Petkov <bp@...en8.de>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        tony.luck@...el.com, marcorr@...gle.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH v8 27/40] x86/boot: Add Confidential Computing type to
 setup_data



On 12/10/21 2:30 PM, Dave Hansen wrote:
> On 12/10/21 12:18 PM, Brijesh Singh wrote:
>> On 12/10/21 1:12 PM, Dave Hansen wrote:
>>> On 12/10/21 7:43 AM, Brijesh Singh wrote:
>>>> +/* AMD SEV Confidential computing blob structure */
>>>> +#define CC_BLOB_SEV_HDR_MAGIC	0x45444d41
>>>> +struct cc_blob_sev_info {
>>>> +	u32 magic;
>>>> +	u16 version;
>>>> +	u16 reserved;
>>>> +	u64 secrets_phys;
>>>> +	u32 secrets_len;
>>>> +	u64 cpuid_phys;
>>>> +	u32 cpuid_len;
>>>> +};
>>> This is an ABI structure rather than some purely kernel construct, right?
>>
>> This is ABI between the guest BIOS and Guest OS. It is defined in the OVMF.
>>
>> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Ftianocore%2Fedk2%2Fblob%2Fmaster%2FOvmfPkg%2FInclude%2FGuid%2FConfidentialComputingSevSnpBlob.h&amp;data=04%7C01%7Cbrijesh.singh%40amd.com%7C460f6abff7f04e065c9108d9bc1bfcf7%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637747650681544593%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&amp;sdata=GI1fAngRJ%2Bj4hcM91UutVXlS1F7kfk2xxtG6I%2BL%2FRYc%3D&amp;reserved=0
>>
>> SEV-SNP FW spec does not have it documented; it's up to the guest BIOS
>> on how it wants to communicate the Secrets and CPUID page location to
>> guest OS.
> 
> Well, no matter where it is defined, could we please make it a bit
> easier for folks to find it in the future?
> 

Noted, I will add a comment so that readers can find it easily. 
Additionally, I will create a doc and get it published on 
developer.amd.com/sev so that information is documented outside the 
source code files.

>>> I searched through all of the specs to which you linked in the cover
>>> letter.  I looked for "blob", "guid", the magic and part of the GUID
>>> itself trying to find where this is defined to see if the struct is correct.
>>>
>>> I couldn't find anything.
>>>
>>> Where is the spec for this blob?  How large is it?  Did you mean to
>>> leave a 4-byte hole after secrets_len and before cpuid_phys?
>> Yes, the length is never going to be > 4GB.
> 
> I was more concerned that this structure could change sizes if it were
> compiled on 32-bit versus 64-bit code.  For kernel ABIs, we try not to
> do that.
> 
> Is this somehow OK when talking to firmware?  Or can a 32-bit OS and
> 64-bit firmware never interact?
> 

For SNP, both the firmware and OS need to be 64-bit. IIRC, both the 
Linux and OVMF do not enable the memory encryption for the 32-bit.

thanks