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Date:   Tue, 18 May 2021 18:36:22 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     Andi Kleen <ak@linux.intel.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@kernel.org>,
        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>, linux-kernel@vger.kernel.org
Subject: Re: [RFC v2-fix 1/1] x86/tdx: Handle in-kernel MMIO
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On Tue, May 18, 2021, Dave Hansen wrote:
> On 5/18/21 10:21 AM, Andi Kleen wrote:
> > Besides instruction decoding works fine for all the existing
> > hypervisors. All we really want to do is to do the same thing as KVM
> > would do.
> 
> Dumb question of the day: If you want to do the same thing that KVM
> does, why don't you share more code with KVM?  Wouldn't you, for
> instance, need to crack the same instruction opcodes?

Pulling in all pf KVM's emulator is a bad idea from a security perspective.  That
could be mitigated to some extent by teaching the emulator to emulate only select
instructions, but it'd still be much higher risk than a barebones guest-specific
implementations.  Because old Intel CPUs don't support unrestricted guest, the set
of instructions that KVM _can_ emulate in total is far, far larger than what is
needed for MMIO.

Allowed instructions aside, KVM needs to handle a large number things a TDX/SEV
guest does not, e.g. segmentation, CPUID model, A/D bit updates, and so on and
so forth.

Refactoring KVM's emulator would also be a monumental task.