Re: folio_mmapped

[David Hildenbrand <david@redhat.com>]

On 28.03.24 11:58, Quentin Perret wrote:
> On Thursday 28 Mar 2024 at 11:32:21 (+0100), David Hildenbrand wrote:
>> ... does that mean that for pKVM with protected VMs, "shared" pages are also
>> never migratable/swappable?
> 
> In our current implementation, yes, KVM keeps its longterm GUP pin on
> pages that are shared back. And we might want to retain this behaviour
> in the short term, even with guest_memfd or using the hybrid approach
> you suggested. But that could totally be relaxed in the future, it's
> "just" a matter of adding extra support to the hypervisor for that. That
> has not been prioritized yet since the number of shared pages in
> practice is relatively small for current use-cases, so ballooning was a
> better option (and in the case of ballooning, we do drop the GUP pin).
> But that's clearly on the TODO list!

Okay, so nothing "fundamental", good!

> 
>> The whole reason I brought up the guest_memfd+memfd pair idea is that you
>> would similarly be able to do the conversion in the kernel, BUT, you'd never
>> be able to mmap+GUP encrypted pages.
>>
>> Essentially you're using guest_memfd for what it was designed for: private
>> memory that is inaccessible.
> 
> Ack, that sounds pretty reasonable to me. But I think we'd still want to
> make sure the other users of guest_memfd have the _desire_ to support
> huge pages,  migration, swap (probably longer term), and related
> features, otherwise I don't think a guest_memfd-based option will
> really work for us :-)

*Probably* some easy way to get hugetlb pages into a guest_memfd would 
be by allocating them for an memfd and then converting/moving them into 
the guest_memfd part of the "fd pair" on conversion to private :)

(but the "partial shared, partial private" case is and remains the ugly 
thing that is hard and I still don't think it makes sense. Maybe it 
could be handles somehow in such a dual approach with some enlightment 
in the fds ... hard to find solutions for things that don't make any 
sense :P )

I also do strongly believe that we want to see some HW-assisted 
migration support for guest_memfd pages. Swap, as you say, maybe in the 
long-term. After all, we're not interested in having MM features for 
backing memory that you could similarly find under Windows 95. Wait, 
that one did support swapping! :P

But unfortunately, that's what the shiny new CoCo world currently 
offers. Well, excluding s390x secure execution, as discussed.

-- 
Cheers,

David / dhildenb