Re: [RFC PATCH 00/18] KVM: Post-copy live migration for guest_memfd

[James Houghton]

On Wed, Jul 10, 2024 at 4:42 PM James Houghton <jthoughton@xxxxxxxxxx> wrote:
> Solution: hook into the gfn -> pfn translation
> ==============================================
>
> The only way to implement post-copy with a non-KVM-specific
> userfaultfd-like system would be to introduce the concept of a
> file-userfault[2] to intercept faults on a guest_memfd.
>
> Instead, we take the simpler approach of adding a KVM-specific API, and
> we hook into the GFN -> HVA or GFN -> PFN translation steps (for
> traditional memslots and for guest_memfd respectively).
>
> I have intentionally added support for traditional memslots, as the
> complexity that it adds is minimal, and it is useful for some VMMs, as
> it can be used to fully implement post-copy live migration.

I want to clarify this sentence a little.

Today, because guest_memfd is only accessed by vCPUs (and is only ever
used for guest-private memory), the concept of "asynchronous
userfaults" isn't exactly necessary. However, when guest_memfd
supports shared memory and KVM is itself able to access it,
asynchronous userfaults become useful in the same way that they are
useful for the non-guest_memfd case.

In a world where guest_memfd requires asynchronous userfaults, adding
support for traditional memslots on top of that is quite simple, and
it somewhat simplies the UAPI.

And for why it is useful for userspace to be able to use KVM Userfault
to implement post-copy live migration, David mentioned this in his
initial RFC[1].

[1]: https://lore.kernel.org/kvm/CALzav=d23P5uE=oYqMpjFohvn0CASMJxXB_XEOEi-jtqWcFTDA@xxxxxxxxxxxxxx/#t