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

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Message-ID: <CADrL8HVxvwB4JrnUf6QtDCyzZojEvR4tr-ELEn+fL8=1cnbMQQ@mail.gmail.com>
Date: Thu, 4 Sep 2025 11:45:30 -0700
From: James Houghton <jthoughton@...gle.com>
To: kalyazin@...zon.com
Cc: Paolo Bonzini <pbonzini@...hat.com>, Sean Christopherson <seanjc@...gle.com>, 
	Oliver Upton <oliver.upton@...ux.dev>, Jonathan Corbet <corbet@....net>, Marc Zyngier <maz@...nel.org>, 
	Yan Zhao <yan.y.zhao@...el.com>, Anish Moorthy <amoorthy@...gle.com>, 
	Peter Gonda <pgonda@...gle.com>, Peter Xu <peterx@...hat.com>, 
	David Matlack <dmatlack@...gle.com>, wei.w.wang@...el.com, kvm@...r.kernel.org, 
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev
Subject: Re: [PATCH v3 00/15] KVM: Introduce KVM Userfault

On Thu, Sep 4, 2025 at 9:43 AM Nikita Kalyazin <kalyazin@...zon.com> wrote:
>
>
>
> On 18/06/2025 05:24, James Houghton wrote:
> > Hi Sean, Paolo, Oliver, + others,
> >
> > Here is a v3 of KVM Userfault. Thanks for all the feedback on the v2,
> > Sean. I realize it has been 6 months since the v2; I hope that isn't an
> > issue.
> >
> > I am working on the QEMU side of the changes as I get time. Let me know
> > if it's important for me to send those patches out for this series to be
> > merged.
>
> Hi Sean and others,
>
> 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. :)

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.

[*]: https://lore.kernel.org/linux-arm-kernel/20250821210042.3451147-1-seanjc@google.com/

>
> Thanks,
> Nikita
>
> [1]:
> https://github.com/firecracker-microvm/firecracker/blob/main/docs/snapshotting/handling-page-faults-on-snapshot-resume.md
> [2]:
> https://github.com/firecracker-microvm/firecracker/tree/feature/secret-hiding
> [3]: https://lore.kernel.org/kvm/20250828093902.2719-1-roypat@amazon.co.uk
> [4]: https://lore.kernel.org/kvm/20250902111951.58315-1-kalyazin@amazon.com
> [5]: https://lore.kernel.org/kvm/20250404154352.23078-1-kalyazin@amazon.com
> [6]:
> https://lore.kernel.org/linux-mm/20250627154655.2085903-1-peterx@redhat.com