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

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Message-ID: <afba42c2-e925-d937-8893-80f995a143b4@linux.intel.com>
Date:   Mon, 12 Jul 2021 17:33:25 -0700
From:   "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
To:     Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.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

Hi Dave,

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?

> 
> That would avoid breaking up the direct mapping.

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer