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Date:   Tue, 18 May 2021 18:17:04 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Andi Kleen <ak@linux.intel.com>
Cc:     Dave Hansen <dave.hansen@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, Andi Kleen wrote:
> > Why does this code exist at all?  TDX and SEV-ES absolutely must share code for
> > handling MMIO reflection.  It will require a fair amount of refactoring to move
> > the guts of vc_handle_mmio() to common code, but there is zero reason to maintain
> > two separate versions of the opcode cracking.
> 
> While that's true on the high level, all the low level details are
> different. We looked at unifying at some point, but it would have been a
> callback hell. I don't think unifying would make anything cleaner.

How hard did you look?  The only part that _must_ be different between SEV and
TDX is the hypercall itself, which is wholly contained at the very end of
vc_do_mmio().

Despite vc_slow_virt_to_phys() taking a pointer to the ghcb, it's unused and
thus the function is 100% generic.

The ghcb->shared_buffer usage throughout the upper levels can be eliminated by
refactoring the stack to take a "u64 *val", since MMIO accesses are currently
bounded to 8 bytes.

> Besides the bulk of the decoding work is already unified in the common x86
> instruction decoder. The actual actions are different, and the code fetching
> is also different 

Huh?  What do you mean by "actual actions"?  Why is the code fetch different?