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

Vlastimil Babka
2021-08-25T09:17:01Z

On 8/24/21 18:42, Joerg Roedel wrote:
> 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.

FTR, this is Sean's reply with concerns in v4:
https://lore.kernel.org/linux-coco/YPCuTiNET%2FhJHqOY@google.com/

I think there are two main arguments there:
- it's not KVM business to decide
- guest may do all page state changes with 2mb granularity so it might be fine
with hugetlb

The latter might become true, but I think it's more probable that sooner
hugetlbfs will learn to split the mappings to base pages - I know people plan to
work on that. At that point qemu will have to recognize if the host kernel is
the new one that can do this splitting vs older one that can't. Preferably
without relying on kernel version number, as backports exist. Thus, trying to
register a hugetlbfs range that either is rejected (kernel can't split) or
passes (kernel can split) seems like a straightforward way. So I'm also in favor
of adding that, hopefuly temporary, check.

Vlastimil

> Regards,
>
> Joerg
>