netdev - Re: [PATCH v2 5/6] platform/x86: intel_tdx_attest: Add TDX Guest attestation interface driver

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Date:   Mon, 12 Jul 2021 17:44:21 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Hans de Goede <hdegoede@...hat.com>,
        Mark Gross <mgross@...ux.intel.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>
Cc:     Peter H Anvin <hpa@...or.com>, Tony Luck <tony.luck@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        platform-driver-x86@...r.kernel.org, bpf@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH v2 5/6] platform/x86: intel_tdx_attest: Add TDX Guest
 attestation interface driver

On 7/12/21 5:33 PM, Kuppuswamy, Sathyanarayanan wrote:

> On 7/8/21 5:38 PM, Andi Kleen wrote:
>>> Expensive and permanently fractures the direct map.
>>> 
>>> I'm struggling to figure out why the direct map is even touched
>>> here.
>> I think Sathya did it this way because the TD interface requires a 
>> physical address.
>>> Why not just use a vmalloc area mapping?  You really just need
>>> *a* decrypted mapping to the page.  You don't need to make
>>> *every* mapping to the page decrypted.
>> 
>> Yes it would be possible to use vmap() on the page and only set
>> the vmap encrypted by passing the right flags directly.
> 
> Is it alright to have non coherent mappings? If yes, any
> documentation reference for it?

Do you mean non-cache-coherent mappings?  I'm not sure what that has to
do with creating "unencrypted" (shared) mappings.

Are you asking exactly which arguments to pass to vmap() or to vmap_pfn()?