linux-kernel - Re: [RFC v2 15/32] x86/tdx: Handle in-kernel MMIO

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Date:   Fri, 7 May 2021 14:52:21 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Tony Luck <tony.luck@...el.com>
Cc:     Andi Kleen <ak@...ux.intel.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@...r.kernel.org
Subject: Re: [RFC v2 15/32] x86/tdx: Handle in-kernel MMIO

On 4/26/21 11:01 AM, Kuppuswamy Sathyanarayanan wrote:
> Handle #VE due to MMIO operations. MMIO triggers #VE with EPT_VIOLATION
> exit reason.

This needs a bit of a history lesson.  "In traditional VMs, MMIO tends
to be implemented by giving a guest access to an mapping which will
cause a VMEXIT on access.  That's not possible in a TDX guest..."

> For now we only handle subset of instruction that kernel uses for MMIO
> oerations. User-space access triggers SIGBUS.

I still don't think that TDX guests should be doing things that they
*KNOW* will cause #VE, including MMIO.  I really want to hear a more
discrete story about why this is the *best* way to do this for Linux
instead of just a hack from the Windows binary driver ecosystem that
seemed expedient.