From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists1p.gnu.org (lists1p.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 24F4AC98328 for ; Mon, 28 Sep 2026 06:04:42 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists1p.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1xB4TB-0004OV-9K; Mon, 28 Sep 2026 02:04:09 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists1p.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1xB4Sz-0004Mg-KF for qemu-devel@nongnu.org; Mon, 28 Sep 2026 02:04:02 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1xB4Sw-0003w4-11 for qemu-devel@nongnu.org; Mon, 28 Sep 2026 02:03:57 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1790575431; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=H6LrWEvTlqKW8tt7imdZdl+NzRhl2ovl7PXVR29UL5E=; b=clCBqtxt/1jaWHtRTy7r1/Yx/dVr4mKt0pnhN9SUOHvuqUIwXFQZYnObCi8+HB4aRLgLYo kqlcrW3gei71prmpLyMHd2d0Jk5B62XV9viwb40A6MqvscC1g4166F1NoyE9BbFtGy5EDp Y1KEieuo9Q4JIdDUVLvM98qHToQbAMw= Received: from mx-prod-mc-03.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-658-l049CIy-PsKX7xvLsEFu1w-1; Mon, 28 Sep 2026 02:03:48 -0400 X-MC-Unique: l049CIy-PsKX7xvLsEFu1w-1 X-Mimecast-MFC-AGG-ID: l049CIy-PsKX7xvLsEFu1w_1790575426 Received: from mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.4]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 6957F1955F29; Mon, 28 Sep 2026 06:03:46 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.44.22.5]) by mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 507D430001A2; Mon, 28 Sep 2026 06:03:45 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id E112621E6A04; Mon, 28 Sep 2026 08:03:42 +0200 (CEST) From: Markus Armbruster To: Michael Roth Cc: , , , , , , , , Fabiano Rosas Subject: Re: [PATCH v6 10/12] hostmem: Support fully shared guest memfd to back a VM In-Reply-To: <20260925015700.1260501-11-michael.roth@amd.com> (Michael Roth's message of "Thu, 24 Sep 2026 20:43:16 -0500") References: <20260925015700.1260501-1-michael.roth@amd.com> <20260925015700.1260501-11-michael.roth@amd.com> Date: Mon, 28 Sep 2026 08:03:42 +0200 Message-ID: <87fqytkj41.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 Received-SPF: pass client-ip=170.10.133.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: 9 X-Spam_score: 0.9 X-Spam_bar: / X-Spam_report: (0.9 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.34, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=-0.01, RCVD_IN_MSPIKE_WL=-0.01, RCVD_IN_SBL_CSS=3.335, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=no autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Michael Roth writes: > From: Peter Xu > > Host backends supports guest-memfd now by detecting whether it's a > confidential VM. There's no way to choose it yet from the memory level to > use it fully shared. If we use guest-memfd, it so far always implies we > need two layers of memory backends, while the guest-memfd only provides the > private set of pages. > > This patch introduces a way so that QEMU can consume guest memfd as the > only source of memory to back the object (aka, fully shared). > > To use the fully shared guest-memfd, one can add a memfd object with: > > -object memory-backend-memfd,x-guest-memfd=on,share=on > > Note that share=on is required with fully shared guest_memfd. > > Since guest-memfd is not yet a drop-in replacement for memfd (e.g. lack > of hugepage support), the option is tagged as experimental for now so > that it can be made available for early enablement/testing without > causing any surprises for other users. > > PS: there's a trivial touch-up on fd<0 check, because the stub to create > guest-memfd may return negative but not -1. > > Reviewed-by: Xiaoyao Li > Reviewed-by: Fabiano Rosas > Reviewed-by: David Hildenbrand > Signed-off-by: Peter Xu > [mdr: add accompanying guest-memfd doc, mark as experimental] > Co-developed-by: Michael Roth > Signed-off-by: Michael Roth [...] > diff --git a/docs/system/guest-memfd.rst b/docs/system/guest-memfd.rst > new file mode 100644 > index 0000000000..f7cef19283 > --- /dev/null > +++ b/docs/system/guest-memfd.rst > @@ -0,0 +1,60 @@ > +.. _guest-memfd: > + > +guest-memfd support > +=================== > + > +Recent kernels allow for the creation of a guest-memfd file > +descriptor, which can be used to back VMs in a similar manner as a > +memfd file descriptor, but is intended specifically for this purpose > +and allows for closer coordination between KVM and the management of > +this memory to enable more advanced/VM-specific use-cases. > + > +Initially this additional functionality centered around providing a > +common/centralized place for managing kernel-side memory handing > +requirements for various Confidential Guest architectures. (For more > +on Confidential Guests, see :ref:`confidential-guest-support`). > + > +guest_memfd has since evolved to become a more general-purpose way to > +allocate/manage guest memory and potentially allow for things like > +providing additional memory isolation within the kernel[1] and support > +for persisting a guest's state across kexec to allow for live-updating > +the host kernel with minimal guest downtime[2]. > + > +Usage > +----- > + > +For Confidential Guests, guest-memfd is currently utilized internally > +by QEMU to handle private guest memory, independently of whatever > +memory backend the user has configured for normal/non-private/shared > +guest memory. To avoid doubling memory, QEMU discards memory in > +response to the guest converting GPA ranges between shared/private. > +(e.g. if GPA X is converted from private to shared, the guest-memfd FD > +offset corresponding to GPA x will be truncated since the memory will > +be provided by the memory backend the user configured for shared > +memory, and vice-versa). This is handled automatically/internally for > +Confidential Guests that rely on this handling and is not directly > +exposed by QEMU command-line options. > + > +For non-Confidential guests, guest-memfd can be used in a manner that > +is somewhat interchangeable with a normal memfd. Currently, this is > +handled by using the same memory-backend implementation as memfd, but > +with an additional 'x-guest-memfd=on' option. E.g.:: > + > + qemu ... \ > + -object memory-backend-memfd,id=ID,size=SIZE,share=on,x-guest-memfd=on > + > +Note that the share=on option is required for guest-memfd, since it > +does not support anonymous memory allocations or COW-like semantics. > + > +Also note that there are a couple of limitations in using guest_memfd > +in this way compared to a normal memfd, which is why the option is > +exposed as an experimental for the time being: > + > + * guest_memfd does not currently support the hugetlb=on option > + * guest_memfd does not currently support Transparent Huge Pages > + > +References > +---------- > + > +- `[1] directmap removal `__ > +- `[2] LUO `__ This is helpful, thanks. > diff --git a/docs/system/index.rst b/docs/system/index.rst > index 4509630fa4..060850a739 100644 > --- a/docs/system/index.rst > +++ b/docs/system/index.rst > @@ -44,3 +44,4 @@ or Hypervisor.Framework. > vm-templating > sriov > qemu-colo > + guest-memfd > diff --git a/qapi/qom.json b/qapi/qom.json > index 5895c97750..602b17ad83 100644 > --- a/qapi/qom.json > +++ b/qapi/qom.json > @@ -780,13 +780,18 @@ > # @seal: if true, create a sealed-file, which will block further > # resizing of the memory (default: true) > # > +# @x-guest-memfd: if 'on', use guest-memfd to back the memory region. > +# See the "guest-memfd" QEMU documentation for more details. Can you make this a link? Try :doc:`/system/guest-memfd`. > +# (default: auto, since: 11.2) What does 'auto' mean? > +# > # Since: 2.12 > ## > { 'struct': 'MemoryBackendMemfdProperties', > 'base': 'MemoryBackendProperties', > 'data': { '*hugetlb': 'bool', > '*hugetlbsize': 'size', > - '*seal': 'bool' }, > + '*seal': 'bool', > + '*x-guest-memfd': 'OnOffAuto' }, You forgot to mark the member unstable. Use { 'name': 'x-guest-memfd', 'features': [ 'unstable' ] } This will make the generator demand documentation. Add this # Features: # # @unstable: Member @x-guest-memfd is experimental. between the member descriptions and "Since:". The 'x-' name prefix is fine, but not required; the feature is the sole source of truth. The prefix can serve as an additional signal to human users. Drawback: you have to rename when the member becomes stable. Up to you. > 'if': 'CONFIG_LINUX' } > > ##