linux-kernel - Re: [PATCH v8 5/5] x86/tdx: Add Quote generation support

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Message-ID: <d0a3d1cc-dfea-0731-c801-97c8eb2cd535@linux.intel.com>
Date:   Thu, 21 Jul 2022 11:57:22 -0700
From:   Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        Isaku Yamahata <isaku.yamahata@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kai Huang <kai.huang@...el.com>,
        Wander Lairson Costa <wander@...hat.com>,
        marcelo.cerri@...onical.com, tim.gardner@...onical.com,
        khalid.elmously@...onical.com, philip.cox@...onical.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 5/5] x86/tdx: Add Quote generation support



On 7/21/22 11:52 AM, Dave Hansen wrote:
> On 7/21/22 11:42, Isaku Yamahata wrote:
>> Regarding to the size of quote, there is no hard limit because it's for future
>> usage.  But here is a rough idea about it.
>>
>> - 4KB(1 page) can accommodate TDREPORT that is defined right now.
>> - 128KB is too small for foreseen use cases.
>> - 1MB is too big. big enough for future.
> 
> Again, we're off in the weeds.
> 
> How does the VMM know how much to read/write?  I have a theory: the spec
> says that R12 is:
> 
> 	"Shared 4KB GPA as input – the memory contains a
> 	TDREPORT_STRUCT."
> 
> That's *A* 4KB GPA.  The maximum is one 4KB page.  That's the only thing
> that makes sense because there's no length in the ABI anywhere.
> 
> What am I missing?

I think you are looking into the old spec. Please check the version
"FEBRUARY 2022"

Following are the ABI details:

R11 - TDG.VP.VMCALL< GetQuote > sub-function per Table 2-3
R12 - Shared GPA as input – the memory contains a TDREPORT_STRUCT. The
       same buffer is used as output – the memory contains a TD Quote.
R13 -  Size of shared GPA. The size must be 4KB-aligned.

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer