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

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Message-ID: <CAPcyv4gBNqDFQEYjWqYTckPg-yy=LrvMw_FNY+tUuEwD35CfyA@mail.gmail.com>
Date:   Tue, 11 May 2021 23:17:10 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        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 Tue, May 11, 2021 at 8:36 AM Dave Hansen <dave.hansen@...el.com> wrote:
>
> 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.

So this circles back to the common problem with the
mwait/monitor/wbinvd patch and this one. "Can't happen" #VE conditions
should be fatal. I.e. have a nice clear message about why the kernel
failed and halt. All the uses of these #VE triggering instructions can
be eliminated ahead of time with auditing and people that load
unaudited out-of-tree modules that trigger #VE get to keep the pieces.
Said pieces will be described to them by the #VE triggered fail
message. This isn't like split lock disable where the code is
difficult to audit.

What am I missing?