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Subject: Re: [RFC v2-fix-v2 2/2] x86/tdx: Ignore WBINVD instruction for TDX
 guest
To:     Dave Hansen <dave.hansen@intel.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@kernel.org>
Cc:     Tony Luck <tony.luck@intel.com>,
        Kirill Shutemov <kirill.shutemov@linux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@kernel.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Raj Ashok <ashok.raj@intel.com>,
        Sean Christopherson <seanjc@google.com>,
        linux-kernel@vger.kernel.org
References: <37ad50ca-f568-4c62-56e2-9e9b1f34084c@linux.intel.com>
 <20210524233211.802033-1-sathyanarayanan.kuppuswamy@linux.intel.com>
 <20210524233211.802033-2-sathyanarayanan.kuppuswamy@linux.intel.com>
 <6d336ac9-72ad-aae5-0b4c-f30a695d198b@intel.com>
From:   Andi Kleen <ak@linux.intel.com>
Message-ID: <76dcb466-225b-760b-525b-6aa8f5c8aae8@linux.intel.com>
Date:   Mon, 24 May 2021 17:39:47 -0700
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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