linux-kernel - Re: [RFC PATCH 0/4] mm, kvm: add guest_memfd support for uffd minor faults

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Message-ID: <b51a9cad-7942-473d-971a-d6fa3b26f63e@amazon.com>
Date: Wed, 19 Nov 2025 17:35:32 +0000
From: Nikita Kalyazin <kalyazin@...zon.com>
To: Peter Xu <peterx@...hat.com>
CC: Mike Rapoport <rppt@...nel.org>, <linux-mm@...ck.org>, Andrea Arcangeli
	<aarcange@...hat.com>, Andrew Morton <akpm@...ux-foundation.org>, Baolin Wang
	<baolin.wang@...ux.alibaba.com>, David Hildenbrand <david@...hat.com>, "Hugh
 Dickins" <hughd@...gle.com>, "Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Michal Hocko <mhocko@...e.com>,
	Paolo Bonzini <pbonzini@...hat.com>, Sean Christopherson <seanjc@...gle.com>,
	Shuah Khan <shuah@...nel.org>, Suren Baghdasaryan <surenb@...gle.com>,
	Vlastimil Babka <vbabka@...e.cz>, <linux-kernel@...r.kernel.org>,
	<kvm@...r.kernel.org>, <linux-kselftest@...r.kernel.org>
Subject: Re: [RFC PATCH 0/4] mm, kvm: add guest_memfd support for uffd minor
 faults



On 17/11/2025 19:39, Peter Xu wrote:
> On Mon, Nov 17, 2025 at 05:55:46PM +0000, Nikita Kalyazin wrote:
>> In our use case, Firecracker snapshot-restore using UFFD [1], we will use
>> UFFD minor/continue to respond to guest_memfd faults in user mappings
>> primarily due to VMM accesses that are required for PV (virtio) device
>> emulation and also KVM accesses when decoding MMIO operations on x86.
> 
> I'm curious if firecracker plans to support live snapshot save.  When with
> something like ioctls_supported flags, guest-memfd can declare support for
> wr-protect support easily too, and synchronous userfaultfd wr-protect traps
> will be an efficient way to do live save.
> 
> I'm guessing it's not an immediate demand now or it would have been asked
> already supporting both MINOR and WP, but I just want to raise this
> question.  Qemu already supports live snapshot save, so it'll always be
> good gmem can also support wp at some point, but it can be done later too.

No, live snapshots haven't been on our plan so far.

> 
> Thanks,
> 
> --
> Peter Xu
>