Re: [PATCH v3 00/15] KVM: Introduce KVM Userfault

[Sean Christopherson <seanjc@google.com>]

On Thu, Sep 04, 2025, James Houghton wrote:
> On Thu, Sep 4, 2025 at 9:43 AM Nikita Kalyazin <kalyazin@amazon.com> wrote:
> > Are there any blockers for merging this series?  We would like to use
> > the functionality in Firecracker for restoring guest_memfd-backed VMs
> > from snapshots via UFFD [1].  [2] is a Firecracker feature branch that
> > builds on top of KVM userfault, along with direct map removal [3], write
> > syscall [4] and UFFD support [5] in guest_memfd (currently in discussion
> > with MM at [6]) series.
> 
> Glad to hear that you need this series. :)

Likewise (though I had slightly-advanced warning from Patrick that Firecracker
wants KVM Userfault).  The main reason I haven't pushed harder on this series is
that I didn't think anyone wanted to use it within the next ~year.

> I am on the hook to get some QEMU patches to demonstrate that KVM
> Userfault can work well with it. I'll try to get that done ASAP now
> that you've expressed interest. The firecracker patches are a nice
> demonstration that this could work too... (I wish the VMM I work on
> was open-source).
> 
> I think the current "blocker" is the kvm_page_fault stuff[*]; KVM
> Userfault will be the first user of this API. I'll review that series
> in the next few days. I'm pretty sure Sean doesn't have any conceptual
> issues with KVM Userfault as implemented in this series.

Yep, Oliver and I (and anyone else that has an opinion) just need to align on the
interface for arch-neutral code.  I think that's mostly on me to spin a v2, and
maybe to show how it all looks when integrated with the userfault stuff.