On Thu, Sep 04, 2025, James Houghton wrote:

On Thu, Sep 4, 2025 at 9:43 AM Nikita Kalyazin [off-list ref] wrote:

quoted

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.