From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B99ED30D406; Mon, 15 Jun 2026 18:30:47 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781548248; cv=none; b=LMSJxgsn5vYjcQZLlqJMp2dCt3MHexo0zE5WkwbTwFPeXBRk61IEypclYfeMENH+aIBGXcnWjcwZ9+t8rIDp1FCnRZG4/q5MLM3QKST0kfEEptURaGMm1CKxb4B0qhPUxYIdPjxf3FmnW23AkfoNA+pmvzYMxBl1CCvb4paAzRk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781548248; c=relaxed/simple; bh=pxQwzcJlcWnEDPGhp6rYbip82v234/yHkMHOg47dTWU=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=kUfOor/pxI0FzoZZY5KyuuVPpizDYqJkVgQnrNlg/nVjuqrYKOTKCnrTgcTV13R+Weozkm5FaOPkNVg5q2048HIqfibSEvQygsYaVWkuo6Aeo8AaOwPZ9NBtYDrOEJPGEmHm8GpIZKHQG/uK9fbahgy/DOLWUz5CuYuBBE4muaY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=k4SHta/B; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="k4SHta/B" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5EB051F000E9; Mon, 15 Jun 2026 18:30:38 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1781548247; bh=5MEuihEHUG8jPWIs1KOkwtZJwTHjAJf8Y63agBr60OI=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=k4SHta/BN4P+7V74eW0MmMMzlqtqnrXT0Gsbh6Nhp0VOwYxab9pF04tz4ZQL/MBsn SMhgGhwip2LeFofAMIBs66wlPK478uWLih57GiC+qFoLG48NNWmtsiea4Mb47lEdjL krT/GtMiQIQMoPHjzxuyVFbAYHv8sf2nX1BUIx576cvKnEPYx7w8u1KqhhFiveuLAK CY9THeOSQLTtFSTB1sadZJWy7xhcXn2Ag3RA+LC2HBEhpPM5XassxK2kNZP7aELp3F PV1byUCpL7cUDATNi4jvlqY8igfyj55/eydhfP4Mk5Qz2jIN9euH0B+Piu6e0Dngzq Loz7d/qD2J04A== Message-ID: Date: Mon, 15 Jun 2026 20:30:36 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RFC 0/3] KVM: guest_memfd: folio migration for non-confidential VMs To: Alexandru Elisei , Shivank Garg Cc: "Matthew Wilcox (Oracle)" , Jan Kara , Andrew Morton , Vlastimil Babka , Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Johannes Weiner , Zi Yan , Matthew Brost , Joshua Hahn , Rakie Kim , Byungchul Park , Gregory Price , Ying Huang , Alistair Popple , Paolo Bonzini , Shuah Khan , Chao Peng , Nikunj A Dadhania , Ira Weiny , Michael Roth , Pankaj Gupta , Ackerley Tng , Fuad Tabba , Sean Christopherson , Vishal Annapurve , Nikita Kalyazin , Patrick Roy , Pratik Sampat , Ashish Kalra , linux-fsdevel@vger.kernel.org, linux-coco@lists.linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, linux-kselftest@vger.kernel.org References: <20260611-shivank-gmem-migrate-v1-0-2d266bfc6f95@amd.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 6/15/26 12:43, Alexandru Elisei wrote: > Hi, > > On Thu, Jun 11, 2026 at 01:05:07PM +0000, Shivank Garg wrote: >> guest_memfd folios are currently marked unmovable, so the kernel cannot >> perform NUMA-balancing, memory compaction, etc. This is unavoidable for >> confidential VMs (SEV-SNP, TDX), since memory is encrypted and copying it >> needs firmware assistance. However, for non-confidential VMs (like >> Firecracker), we can migrate the folios. >> >> This series enables folio migration for non-confidential guest_memfd and >> also lays the groundwork for migrating confidential guest_memfd later. >> Once firmware-assisted copying support is available, those VMs can be >> made movable, the confidential folio content can be copied separately, >> and the destination folio marked with FOLIO_CONTENT_COPIED so >> __migrate_folio() skips the host-side folio_mc_copy(). > > I always thought that one of the nice things about using guest_memfd as a > memory backend, as opposed to host userspace mappings, is that the host > cannot unmap VM memory because of KSM, automatic NUMA balancing, hugepage > collapse, compaction, etc, acting on the host userspace mapping of the > VM memory, and outside of the VMM's or KVM's control. Yeah, but it doesn't play nice with THPs / large folios. So if you want to run something else on a hypervisor than just confidential VMs, you definitely want guest_memfd to be as nice to the system. That is, support page migration if nothing speaks against it. Now, if something speaks against it, for sure we can just leave the pages be unmovable. Fortunately, the patch is rather trivial. -- Cheers, David