From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) (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 3061E3B992F for ; Tue, 2 Jun 2026 08:22:12 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.133.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780388534; cv=none; b=WpTR4+bwFfXpSSlicZu12ohedAu3p1EJq65EL9DE1VyLCcukrUOFlrLUM0FpeyrLgXTM6kF5cLgipRRimK5BoP5aFdnOTBjBH2WNtOieUql3bUCuPRpBvIcIG7XygV8mcrm9sqCCg9SF6pfpbfsdF6MkFO2MmnpZh5IJoyvEw1Y= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780388534; c=relaxed/simple; bh=blH/TH3DPDlZMNnrL7AMZ3qp6Ajm0tsbE6gqdjkETxU=; h=From:To:Cc:Subject:In-Reply-To:References:Date:Message-ID: MIME-Version:Content-Type; b=Wfs++rm1blRzHySsDG9g7fdbO/9lyU0Qr+6xSdb2ja9yK3jkd87tYn1bY2wdrenNM0ugGpl9j/G5zShGgEv8qUBSSd4homSU3OvJiFOLJF/6wmVLBEnybrXhvkKF/1eN5GjMKHPLueuDato4zf3WA+RJ8ZrqYkNHg5e9S2oYFeM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=UcvSTlGD; arc=none smtp.client-ip=170.10.133.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="UcvSTlGD" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1780388532; 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=YVaJ28YncjhCLTaSyTtA/MThSayBHlneCG9Gaac4gfA=; b=UcvSTlGDhcMUqrkrZqc80iqBm0tQuoERdZvbRTWZsQVzFSc7nRtWcfJp8+yNWu+vrjMbuy 83m99i+OCnBDa3h9+EdYGFv0k7wgKkyHUCb2GTw9EUB/tA/9LZPI1p+fBn9xo5iPkaGa1E WAc10ZRiLZMA4XIz1wW0to0RTJ7+rlM= Received: from mx-prod-mc-06.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-154-SkdbMs3PMu2HihVbiuwNMQ-1; Tue, 02 Jun 2026 04:22:06 -0400 X-MC-Unique: SkdbMs3PMu2HihVbiuwNMQ-1 X-Mimecast-MFC-AGG-ID: SkdbMs3PMu2HihVbiuwNMQ_1780388525 Received: from mx-prod-int-10.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-10.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.95]) (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-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 73B8018005B8; Tue, 2 Jun 2026 08:22:04 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.44.22.2]) by mx-prod-int-10.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 8A71C1681; Tue, 2 Jun 2026 08:22:03 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 16CE721E6A01; Tue, 02 Jun 2026 10:22:01 +0200 (CEST) From: Markus Armbruster To: Michael Roth Cc: , , , , , , , , , , Subject: Re: [PATCH RFC 02/12] hostmem: Introduce dedicated memory backend for guest_memfd In-Reply-To: <20260528000416.8161-3-michael.roth@amd.com> (Michael Roth's message of "Wed, 27 May 2026 19:03:27 -0500") References: <20260528000416.8161-1-michael.roth@amd.com> <20260528000416.8161-3-michael.roth@amd.com> Date: Tue, 02 Jun 2026 10:22:01 +0200 Message-ID: <871pep73o6.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 3.6 on 10.30.177.95 Michael Roth writes: > In the initial implementation of guest_memfd in the linux kernel, it > was not possible to map memory into userspace for direct access; instead > the memory provided by the memory backend would be used for cases where > a confidential VM wants to access normal/unprotected/unencrypted memory > that can be used for shared memory use cases, and for access to private > memory a guest_memfd could be associated with the same memslot. A memory > 'private' attribute set via KVM_SET_MEMORY_ATTRIBUTES could then be used > to have KVM route to the approprate backing memory. > > In that model, it didn't make sense to introduce a specific backend for > guest_memfd, since there was always a generally need to have a separate a general need? > backend type to handle shared memory access/allocation. Instead, QEMU > configures the guest_memfd support for the associated memslots > internally for cases where it is running a confidential VM. > > However, with recent changes in guest_memfd kernel support, it is now > possible to mmap() a guest_memfd FD into userspace and use it for shared > memory, as well as continue to use the same physical pages for the same > GPA ranges after they are converted to private ("in-place conversion"). > > To enable the use of this mmap()-able/guest_memfd-provided memory to be > used for normal/shared memory instead of just for private memory, > introduce a dedicated guest_memfd memory backend that can be used both > for confidential VMs that wish to make use of in-place conversion, as > well as for non-confidential VMs that just want to make use of > guest_memfd for normal memory (which can be useful both for testing as > well as a stepping stone to things like software-protected VMs where the > host can be trusted to provided some additional degree of isolation for > the VM independently of hardware support). > > Signed-off-by: Michael Roth [...] > diff --git a/qapi/qom.json b/qapi/qom.json > index dd45ac1087..502fafeb15 100644 > --- a/qapi/qom.json > +++ b/qapi/qom.json > @@ -661,7 +661,8 @@ > # @share: if false, the memory is private to QEMU; if true, it is > # shared (default false for backends memory-backend-file and > # memory-backend-ram, true for backends memory-backend-epc, > -# memory-backend-memfd, and memory-backend-shm) > +# memory-backend-memfd, memory-backend-shm, and > +# memory-backend-guest-memfd) > # > # @reserve: if true, reserve swap space (or huge pages) if applicable > # (default: true) (since 6.1) > @@ -780,6 +781,18 @@ > '*seal': 'bool' }, > 'if': 'CONFIG_LINUX' } > > +## > +# @MemoryBackendGuestMemfdProperties: > +# > +# Properties for memory-backend-guest-memfd objects. > +# > +# Since: 11.1 > +## > +{ 'struct': 'MemoryBackendGuestMemfdProperties', > + 'base': 'MemoryBackendProperties', > + 'data': {}, > + 'if': 'CONFIG_LINUX' } > + Identical to MemoryBackendProperties so far. > ## > # @MemoryBackendShmProperties: > # > @@ -1234,6 +1247,8 @@ > 'memory-backend-file', > { 'name': 'memory-backend-memfd', > 'if': 'CONFIG_LINUX' }, > + { 'name': 'memory-backend-guest-memfd', > + 'if': 'CONFIG_LINUX' }, > 'memory-backend-ram', > { 'name': 'memory-backend-shm', > 'if': 'CONFIG_POSIX' }, > @@ -1312,6 +1327,8 @@ > 'memory-backend-file': 'MemoryBackendFileProperties', > 'memory-backend-memfd': { 'type': 'MemoryBackendMemfdProperties', > 'if': 'CONFIG_LINUX' }, > + 'memory-backend-guest-memfd': { 'type': 'MemoryBackendGuestMemfdProperties', > + 'if': 'CONFIG_LINUX' }, You could use MemoryBackendProperties here, and drop MemoryBackendGuestMemfdProperties, similar to how memory-backend-ram is done. > 'memory-backend-ram': 'MemoryBackendProperties', > 'memory-backend-shm': { 'type': 'MemoryBackendShmProperties', > 'if': 'CONFIG_POSIX' }, Should we provide guidance on when to use which memory backend? The commit message provides some clues... > diff --git a/qemu-options.hx b/qemu-options.hx > index 96ae41f787..3c754c149f 100644 > --- a/qemu-options.hx > +++ b/qemu-options.hx > @@ -5858,6 +5858,11 @@ SRST > off will cause a failure during allocation because it is not supported > by this backend. > > + ``-object memory-backend-guest-memfd,id=id,prealloc=on|off,size=size,host-nodes=host-nodes,policy=default|preferred|bind|interleave`` > + Creates an anonymous memory file backend object that has similar > + semantics to memfd, but is also usable as private memory when > + running as a confidential VM. (Linux only) There is no object type "memfd". Do you mean "memory-backend-memfd"? If yes, that one has additional properties @hugetlb, @hugetlbsize, and @seal. Why are they not needed for memory-backend-guest-memfd? > + > ``-object iommufd,id=id[,fd=fd]`` > Creates an iommufd backend which allows control of DMA mapping > through the ``/dev/iommu`` device.