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

From: Dan Williams
Date: Tue May 11 2021 - 11:44:13 EST

On Tue, May 11, 2021 at 8:36 AM Dave Hansen <dave.hansen@xxxxxxxxx> 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.

Thanks Dave, this really helps.