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

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Message-ID: <CAPcyv4hz9WFKbEymPHfMpZSi0RGn9rEv5eAx2Y+9EsfXTdv3YQ@mail.gmail.com>
Date:   Mon, 24 May 2021 17:53:54 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Tony Luck <tony.luck@...el.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 Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC v2-fix-v2 2/2] x86/tdx: Ignore WBINVD instruction for TDX guest

On Mon, May 24, 2021 at 5:40 PM Andi Kleen <ak@...ux.intel.com> wrote:
>
>
> 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.
>

I have no problem pointing at KVM as to why the risk is mitigated, but
I do have a problem with misrepresenting the scope of the risk.