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

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Message-ID: <aRt52tDCh72ytIp1@x1.local>
Date: Mon, 17 Nov 2025 14:39:06 -0500
From: Peter Xu <peterx@...hat.com>
To: Nikita Kalyazin <kalyazin@...zon.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 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.

Thanks,

-- 
Peter Xu