Re: RFC: A KVM-specific alternative to UserfaultFD

[David Matlack <dmatlack@google.com>]

On Thu, Nov 9, 2023 at 10:33 AM David Matlack <dmatlack@google.com> wrote:
> On Thu, Nov 9, 2023 at 9:58 AM Sean Christopherson <seanjc@google.com> wrote:
> > For both cases, KVM will need choke points on all accesses to guest memory.  Once
> > the choke points exist and we have signed up to maintain them, the extra burden of
> > gracefully handling "missing" memory versus frozen memory should be relatively
> > small, e.g. it'll mainly be the notify-and-wait uAPI.
>
> To be honest, the choke points are a relatively small part of any
> KVM-based demand paging scheme. We still need (a)-(e) from my original
> email.

Another small thing here: I think we can find clean choke point(s)
that fit both freezing and demand paging (aka "missing" pages), but
there is a difference to keep in mind. To freeze guest memory KVM only
needs to return an error at the choke point(s). Whereas handling
"missing" pages may require blocking, which adds constraints on where
the choke point(s) can be placed.