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Mon, 08 Jun 2026 06:40:19 -0400 X-MC-Unique: Xg2CV7XdPcCORuKnFj17Mw-1 X-Mimecast-MFC-AGG-ID: Xg2CV7XdPcCORuKnFj17Mw_1780915217 Received: from mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.12]) (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-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 3AE03180064A; Mon, 8 Jun 2026 10:40:17 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.44.22.28]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 73C7019560A2; Mon, 8 Jun 2026 10:40:16 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 1040121E6A01; Mon, 08 Jun 2026 12:40:14 +0200 (CEST) Resent-To: ashish.kalra@amd.com, michael.roth@amd.com, pankaj.gupta@amd.com, ackerleytng@google.com, isaku.yamahata@intel.com, xiaoyao.li@intel.com, david@kernel.org, chao.p.peng@linux.intel.com, qemu-devel@nongnu.org, kvm@vger.kernel.org Resent-From: Markus Armbruster Resent-Date: Mon, 08 Jun 2026 12:40:14 +0200 Resent-Message-ID: <87v7btiacx.fsf@pond.sub.org> From: Markus Armbruster To: Michael Roth Cc: Markus Armbruster , , , , , , , , , , , Subject: Re: [PATCH RFC 02/12] hostmem: Introduce dedicated memory backend for guest_memfd In-Reply-To: (Michael Roth's message of "Wed, 3 Jun 2026 01:19:32 -0500") References: <20260528000416.8161-1-michael.roth@amd.com> <20260528000416.8161-3-michael.roth@amd.com> <871pep73o6.fsf@pond.sub.org> Date: Mon, 08 Jun 2026 10:20:22 +0200 Message-ID: <87v7btl9yx.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 3.0 on 10.30.177.12 Michael Roth writes: > On Tue, Jun 02, 2026 at 10:22:01AM +0200, Markus Armbruster wrote: >> 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? > > Much nicer :) > >> >> > 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. > > That's true. I think I was anticipating it being warranted at some point, but > that doesn't need to happen here. > >> >> > '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... > > Were you thinking from a schema perspective, or something more > user-facing? The QAPI schema doc comments become the QEMU QMP Reference Manual, which I believe is the first stop for "how do I use this?" Sometimes, a full answer just doesn't fit there comfortably. So we put it elsewhere, and point to it from the QMP Reference. > Either way, docs/system/confidential-guest-support.rst could definitely > use some sprucing up as part of this series, so I can cover this aspect > there as well. > >> >> > 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"? > > Yes, will update. > >> >> If yes, that one has additional properties @hugetlb, @hugetlbsize, and >> @seal. Why are they not needed for memory-backend-guest-memfd? > > ATM, hugetlb is not enabled for guest_memfd in the kernel. It's likely the > same set of options will apply, but there are also efforts to do things like > plumb DAX memory through guest_memfd for confidential VMs where maybe we end > up needing to be a bit more flexible/creative... not sure, but it seemed > like a good idea to give ourselves a clean slate since the support isn't > there yet anyway. I gather these properties cannot work today. I agree we shouldn't add them until they do. > For seal, I'm not aware of any plan to support that for guest_memfd, so > it seems like unecessary baggage to pull in. Likewise. > Thanks, > > Mike > >> >> > + >> > ``-object iommufd,id=id[,fd=fd]`` >> > Creates an iommufd backend which allows control of DMA mapping >> > through the ``/dev/iommu`` device. >>