linux-kernel - Re: [RFC v1 00/26] Add TDX Guest Support

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Date:   Sun, 4 Apr 2021 08:02:13 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>
Cc:     Andi Kleen <ak@...ux.intel.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Raj Ashok <ashok.raj@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC v1 00/26] Add TDX Guest Support

It occurred to me that I've been doing a lot of digging in the TDX spec
lately.  I think we can all agree that the "Architecture Specification"
is not the world's easiest, most disgestable reading.  It's hard to
figure out what the Linux relation to the spec is.

One bit of Documentation we need for TDX is a description of the memory
states.  For instance, it would be nice to spell out the different
classes of memory, how they are selected, who selects them, and who
enforces the selection.  What faults are generated on each type and who
can induce those?

For instance:

TD-Private memory is selected by the Shared/Private bit in Present=1
guest PTEs.  When the hardware page walker sees that bit, it walk the
secure EPT.  The secure EPT entries can only be written by the TDX
module, although they are written at the request of the VMM.  The TDX
module enforces rules like ensuring that the memory mapped by secure EPT
is not mapped multiple times.  The VMM can remove entries.  From the
guest perspective, all private memory accesses are either successful, or
result in a #VE.  Private memory access does not cause VMExits.

Would that be useful to folks?