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

[Sean Christopherson]

On Tue, Aug 31, 2021, David Hildenbrand wrote:
> 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 wasn't referring to code churn.  By "invasive" I mean number of touchpoints in
KVM as well as the nature of the touchpoints.  E.g. poking into how KVM uses
available bits in its shadow PTEs and adding multiple checks through KVM's page
fault handler, versus two callbacks to get the PFN and page size.

> 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.

Replacing a private page with a shared page for a given GFN is very much a
requirement as it's expected behavior for all VMM+guests when converting guest
memory between shared and private.

Punching holes is a sort of optional requirement.  It's a "requirement" in that
it's allowed if the backing store supports such a behavior, optional in that
support wouldn't be strictly necessary and/or could come with constraints.  The
expected use case is that host userspace would punch a hole to free unreachable
private memory, e.g. after the corresponding GFN(s) is converted to shared, so
that it doesn't consume 2x memory for the guest.