linux-kernel - Re: [RFC v2-fix-v2 2/2] x86/tdx: Ignore WBINVD instruction for TDX guest

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Date:   Mon, 24 May 2021 17:39:47 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>
Cc:     Tony Luck <tony.luck@...el.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 v2-fix-v2 2/2] x86/tdx: Ignore WBINVD instruction for TDX
 guest


On 5/24/2021 4:42 PM, Dave Hansen wrote:
> On 5/24/21 4:32 PM, Kuppuswamy Sathyanarayanan wrote:
>> Functionally only DMA devices can notice a side effect from
>> WBINVD's cache flushing.
> This seems to be trying to make some kind of case that the only visible
> effects from WBINVD are for DMA devices.  That's flat out wrong.  It
> might be arguable that none of the other cases exist in a TDX guest, but
> it doesn't excuse making such a broad statement without qualification.

We're describing a few sentences down that guests run with EPT 
IgnorePAT=1, which is the qualification.

>
> Just grep in the kernel for a bunch of reasons this is wrong.
>
> Where did this come from?

Again the logic is very simple: TDX guest code is (mostly) about 
replacing KVM code with in kernel code, so we're just doing the same as 
KVM. You cannot get any more proven than that.


-Andi