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SubjectRe: [PATCHv3 3/3] x86/tdx: Handle load_unaligned_zeropad() page-cross to a shared page
On Thu, May 26, 2022 at 09:20:56AM -0700, Dave Hansen wrote:
> On 5/24/22 15:10, Kirill A. Shutemov wrote:
> > + /*
> > + * MMIO accesses suppose to be naturally aligned and therefore never
> > + * cross a page boundary. Seeing unaligned accesses indicates a bug or
> > + * load_unaligned_zeropad() that steps into unmapped shared page.
>
> Wait a sec though...
>
> We've been talking all along about how MMIO accesses are in some cases
> just plain old compiler-generated memory accesses. It's *probably* bad
> code that does this, but it's not necessarily a bug.

Compiler-generated memory accesses tend to be aligned too. You need to do
something make them unalinged, like __packed or pointer trickery.

> It's kinda like the split lock detection patches. Those definitely
> found some stupid stuff, but it wasn't anything that I would have called
> an outright bug. Plus, in those cases, folks had explicitly opted in to
> more crashes on stupid stuff.
>
> That stupid stuff _might_ be rare enough that it's still OK to just punt
> on it and not emulate the instruction (aka. crash). Or, to say that TDX
> guests are opting in to being more fragile, just like with split lock
> detection.

I think it is reasonable to expect that TDX user value its data security
higher than uptime.

And I'm not sure that compare unaligned MMIO access to split-lock is fair.
Split-lock is performance hit, but semantics is defined. In unalgined MMIO
case, I think the behaviour is not defined: it is not clear what memory
reqested should be issued on the memory bus in case of byte-algined 4-byte
access. It can make a difference on device side.

> But, either of those would call for a very different comment.

Fair enough.

--
Kirill A. Shutemov

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