Re: [PATCH v5 00/13] KVM: mm: fd-based approach for supporting KVM guest private memory

["Andy Lutomirski" <luto@kernel.org>]

On Wed, Mar 30, 2022, at 10:58 AM, Sean Christopherson wrote:
> On Wed, Mar 30, 2022, Quentin Perret wrote:
>> On Wednesday 30 Mar 2022 at 09:58:27 (+0100), Steven Price wrote:
>> > On 29/03/2022 18:01, Quentin Perret wrote:
>> > > Is implicit sharing a thing? E.g., if a guest makes a memory access in
>> > > the shared gpa range at an address that doesn't have a backing memslot,
>> > > will KVM check whether there is a corresponding private memslot at the
>> > > right offset with a hole punched and report a KVM_EXIT_MEMORY_ERROR? Or
>> > > would that just generate an MMIO exit as usual?
>> > 
>> > My understanding is that the guest needs some way of tagging whether a
>> > page is expected to be shared or private. On the architectures I'm aware
>> > of this is done by effectively stealing a bit from the IPA space and
>> > pretending it's a flag bit.
>> 
>> Right, and that is in fact the main point of divergence we have I think.
>> While I understand this might be necessary for TDX and the likes, this
>> makes little sense for pKVM. This would effectively embed into the IPA a
>> purely software-defined non-architectural property/protocol although we
>> don't actually need to: we (pKVM) can reasonably expect the guest to
>> explicitly issue hypercalls to share pages in-place. So I'd be really
>> keen to avoid baking in assumptions about that model too deep in the
>> host mm bits if at all possible.
>
> There is no assumption about stealing PA bits baked into this API.  Even within
> x86 KVM, I consider it a hard requirement that the common flows not assume the
> private vs. shared information is communicated through the PA.

Quentin, I think we might need a clarification.  The API in this patchset indeed has no requirement that a PA bit distinguish between private and shared, but I think it makes at least a weak assumption that *something*, a priori, distinguishes them.  In particular, there are private memslots and shared memslots, so the logical flow of resolving a guest memory access looks like:

1. guest accesses a GVA

2. read guest paging structures

3. determine whether this is a shared or private access

4. read host (KVM memslots and anything else, EPT, NPT, RMP, etc) structures accordingly.  In particular, the memslot to reference is different depending on the access type.

For TDX, this maps on to the fd-based model perfectly: the host-side paging structures for the shared and private slots are completely separate.  For SEV, the structures are shared and KVM will need to figure out what to do in case a private and shared memslot overlap.  Presumably it's sufficient to declare that one of them wins, although actually determining which one is active for a given GPA may involve checking whether the backing store for a given page actually exists.

But I don't understand pKVM well enough to understand how it fits in.  Quentin, how is the shared vs private mode of a memory access determined?  How do the paging structures work?  Can a guest switch between shared and private by issuing a hypercall without changing any guest-side paging structures or anything else?

It's plausible that SEV and (maybe) pKVM would be better served if memslots could be sparse or if there was otherwise a direct way for host userspace to indicate to KVM which address ranges are actually active (not hole-punched) in a given memslot or to otherwise be able to make a rule that two different memslots (one shared and one private) can't claim the same address.