Re: [PATCH Part2 v5 08/45] x86/fault: Add support to handle the RMP fault for user address

Joerg Roedel
2021-08-24T16:42:53Z

On Mon, Aug 23, 2021 at 07:50:22AM -0700, Dave Hansen wrote:
> It *has* to be done in KVM, IMNHO.
>
> The core kernel really doesn't know much about SEV. It *really* doesn't
> know when its memory is being exposed to a virtualization architecture
> that doesn't know how to split TLBs like every single one before it.
>
> This essentially *must* be done at the time that the KVM code realizes
> that it's being asked to shove a non-splittable page mapping into the
> SEV hardware structures.
>
> The only other alternative is raising a signal from the fault handler
> when the page can't be split. That's a *LOT* nastier because it's so
> much later in the process.
>
> It's either that, or figure out a way to split hugetlbfs (and DAX)
> mappings in a failsafe way.

Yes, I agree with that. KVM needs a check to disallow HugeTLB pages in
SEV-SNP guests, at least as a temporary workaround. When HugeTLBfs
mappings can be split into smaller pages the check can be removed.

Regards,

Joerg