Re: [RFC] KVM: mm: fd-based approach for supporting KVM guest private memory

[David Hildenbrand]

On 28.08.21 00:28, Sean Christopherson wrote:
> On Fri, Aug 27, 2021, Andy Lutomirski wrote:
> > On Thu, Aug 26, 2021, at 2:26 PM, David Hildenbrand wrote:
> > > On 26.08.21 19:05, Andy Lutomirski wrote:
> >
> > > > Oof.  That's quite a requirement.  What's the point of the VMA once all
> > > > this is done?
> > >
> > > You can keep using things like mbind(), madvise(), ... and the GUP code
> > > with a special flag might mostly just do what you want. You won't have
> > > to reinvent too many wheels on the page fault logic side at least.
>
> Ya, Kirill's RFC more or less proved a special GUP flag would indeed Just Work.
> However, the KVM page fault side of things would require only a handful of small
> changes to send private memslots down a different path.  Compared to the rest of
> the enabling, it's quite minor.
>
> The counter to that is other KVM architectures would need to learn how to use the
> new APIs, though I suspect that there will be a fair bit of arch enabling regardless
> of what route we take.
>
> > You can keep calling the functions.  The implementations working is a
> > different story: you can't just unmap (pte_numa-style or otherwise) a private
> > guest page to quiesce it, move it with memcpy(), and then fault it back in.
>
> Ya, I brought this up in my earlier reply.  Even the initial implementation (without
> real NUMA support) would likely be painful, e.g. the KVM TDX RFC/PoC adds dedicated
> logic in KVM to handle the case where NUMA balancing zaps a _pinned_ page and then
> KVM fault in the same pfn.  It's not thaaat ugly, but it's arguably more invasive
> to KVM's page fault flows than a new fd-based private memslot scheme.

I might have a different mindset, but less code churn doesn't 
necessarily translate to "better approach".

I'm certainly not pushing for what I proposed (it's a rough, broken 
sketch). I'm much rather trying to come up with alternatives that try 
solving the same issue, handling the identified requirements.

I have a gut feeling that the list of requirements might not be complete 
yet. For example, I wonder if we have to protect against user space 
replacing private pages by shared pages or punishing random holes into 
the encrypted memory fd.

--
Thanks,

David / dhildenb