linux-kernel - Re: [RFC v2 14/32] x86/tdx: Handle port I/O

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Date:   Tue, 11 May 2021 08:35:44 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Dan Williams <dan.j.williams@...el.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Tony Luck <tony.luck@...el.com>,
        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 Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC v2 14/32] x86/tdx: Handle port I/O

On 5/10/21 2:57 PM, Dan Williams wrote:
>> Decompression code uses port IO for earlyprintk. We must use
>> paravirt calls there too if we want to allow earlyprintk.
> What is the tradeoff between teaching the decompression code to handle
> #VE (the implied assumption) vs teaching it to avoid #VE with direct
> TDVMCALLs (the chosen direction)?

To me, the tradeoff is not just "teaching" the code to handle a #VE, but
ensuring that the entire architecture works.

Intentionally invoking a #VE is like making a function call that *MIGHT*
recurse on itself.  Sure, you can try to come up with a story about
bounding the recursion.  But, I don't see any semblance of that in this
series.

Exception-based recursion is really nasty because it's implicit, not
explicit.  That's why I'm advocating for a design where the kernel never
intentionally causes a #VE: it never intentionally recurses without bounds.