On 7/26/22 03:21, Borislav Petkov wrote: > On Tue, Jun 14, 2022 at 03:02:27PM +0300, Kirill A. Shutemov wrote: >> But, this approach does not work for unaccepted memory. For TDX, a load >> from unaccepted memory will not lead to a recoverable exception within >> the guest. The guest will exit to the VMM where the only recourse is to >> terminate the guest. > FTR, this random-memory-access-to-unaccepted-memory-is-deadly thing is > really silly. We should be able to handle such cases - because they do > happen often - in a more resilient way. Just look at the complex dance > this patch needs to do just to avoid this. > > IOW, this part of the coco technology needs improvement. This particular wound is self-inflicted. The hardware can *today* generate a #VE for these accesses. But, to make writing the #VE code more straightforward, we asked that the hardware not even bother delivering the exception. At the time, nobody could come up with a case why there would ever be a legitimate, non-buggy access to unaccepted memory. We learned about load_unaligned_zeropad() the hard way. I never ran into it and never knew it was there. Dangit. We _could_ go back to the way it was originally. We could add load_unaligned_zeropad() support to the #VE handler, and there's little risk of load_unaligned_zeropad() itself being used in the interrupts-disabled window early in the #VE handler. That would get rid of all the nasty adjacent page handling in the unaccepted memory code. But, that would mean that we can land in the #VE handler from more contexts. Any normal, non-buggy use of load_unaligned_zeropad() can end up there, obviously. We would, for instance, need to be more careful about #VE recursion. We'd also have to make sure that _bugs_ that land in the #VE handler can still be handled in a sane way. To sum it all up, I'm not happy with the complexity of the page acceptance code either but I'm not sure that it's bad tradeoff compared to greater #VE complexity or fragility. Does anyone think we should go back and really reconsider this?
