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

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Message-ID: <8408090a-d2cc-4b90-99fe-183d49081ea4@amazon.com>
Date: Tue, 23 Sep 2025 13:13:51 +0100
From: Nikita Kalyazin <kalyazin@...zon.com>
To: Sean Christopherson <seanjc@...gle.com>, James Houghton
	<jthoughton@...gle.com>
CC: Paolo Bonzini <pbonzini@...hat.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 05/09/2025 13:27, Sean Christopherson wrote:
> On Thu, Sep 04, 2025, James Houghton wrote:
>> On Thu, Sep 4, 2025 at 9:43 AM Nikita Kalyazin <kalyazin@...zon.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.\

Sounds good, thanks.  Do you think you'll be having time to work on the 
v2 soonish?  Is defining and implementing the interface a strict 
prerequisite for this series?