linux-kernel - Re: [RFC v2-fix-v3 1/1] x86/tdx: Skip WBINVD instruction for TDX guest

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Message-ID: <ec55256c-3a44-5265-fea8-018a229e92da@intel.com>
Date:   Tue, 8 Jun 2021 15:17:08 -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>,
        Tony Luck <tony.luck@...el.com>,
        Dan Williams <dan.j.williams@...el.com>
Cc:     Andi Kleen <ak@...ux.intel.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        Raj Ashok <ashok.raj@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC v2-fix-v3 1/1] x86/tdx: Skip WBINVD instruction for TDX
 guest

On 6/8/21 2:35 PM, Kuppuswamy Sathyanarayanan wrote:
> Persistent memory is also currently not supported. Another code
> path that uses WBINVD is the MTRR driver, but EPT/virtualization
> always disables MTRRs so those are not needed. This all implies
> WBINVD is not needed with current TDX.

It's one thing to declare something unsupported.  It's quite another to
declare it unsupported and then back it up with code to ensure that any
attempted use is thwarted.

This patch certainly shows us half of the solution.  But, to be
complete, we also need to see the other half: where is the patch or
documentation for why it is not *possible* to encounter persistent
memory in a TDX guest?

BTW, "persistent memory" is much more than Intel Optane DCPMM.