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Iglesias" To: Demi Marie Obenour Subject: Re: virtio-msg inter-VM transport vs virtio-vhost-user Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/2.2.14+84 (2efcabc4) (2026-01-25) X-Originating-IP: [10.180.168.240] X-ClientProxiedBy: satlexmb07.amd.com (10.181.42.216) To satlexmb07.amd.com (10.181.42.216) X-EOPAttributedMessage: 0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: BN2PEPF000055E0:EE_|DS7PR12MB6310:EE_ X-MS-Office365-Filtering-Correlation-Id: be420ef1-cc3e-4f57-e529-08de7a074b28 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|82310400026|7416014|376014|36860700016|1800799024; X-Microsoft-Antispam-Message-Info: 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 X-Forefront-Antispam-Report: CIP:165.204.84.17;CTRY:US;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:satlexmb07.amd.com;PTR:InfoDomainNonexistent;CAT:NONE;SFS:(13230040)(82310400026)(7416014)(376014)(36860700016)(1800799024);DIR:OUT;SFP:1101; X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: Rebx48edpJhC1GaHPXy/YvEqY83uMHfPT9a29lA+tNpkazyTGEF+rWtJYB9ZDWL9W08qwJofhyWklf0R6ud9Ht05qe3oIvKB0PP9vL5/lM1kzDwDWBfUO4RUuX5ES4lLdbjAK3Zy/gNRHD6+7Sy4059A3iCt6glsNZbNTOzVCSy4j+ZcZwnBzwkXmxdoTOpEBFexEmPJ+86YHRvZwgMboiLzxdvhb/ztszjJ+6Z28u0TxPWf7hooxBMhqUlUdVHjN5Vpy9QyQbpT9y49YKofpLqDSEApObXB6EzUq0/FD3oQM71Jm/UqlSGl64R58BQTG8XxcmM4OBL6/LVvCMWgUFfv4skkdiOesGIT42lXFQKkyxECfWXL2Jp+kgeOBuhph0+b49bbUlfnMRNxTv4cXuFMfeW2OvgB0+3U8sNTivlbWMILFMSL+NMHST2ZUH4O X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 04 Mar 2026 16:01:27.7115 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: be420ef1-cc3e-4f57-e529-08de7a074b28 X-MS-Exchange-CrossTenant-Id: 3dd8961f-e488-4e60-8e11-a82d994e183d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=3dd8961f-e488-4e60-8e11-a82d994e183d;Ip=[165.204.84.17];Helo=[satlexmb07.amd.com] X-MS-Exchange-CrossTenant-AuthSource: BN2PEPF000055E0.namprd21.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: DS7PR12MB6310 Message-ID-Hash: SHPD7XIFUAPZANBX6W6B46XJSUQ5DH5N X-Message-ID-Hash: SHPD7XIFUAPZANBX6W6B46XJSUQ5DH5N X-MailFrom: Edgar.Iglesias@amd.com X-Mailman-Rule-Misses: dmarc-mitigation; no-senders; approved; emergency; loop; banned-address; member-moderation; header-match-devel.spectrum-os.org-0; header-match-devel.spectrum-os.org-1; header-match-devel.spectrum-os.org-2; header-match-devel.spectrum-os.org-3; header-match-devel.spectrum-os.org-4; nonmember-moderation; administrivia; implicit-dest; max-recipients; max-size; news-moderation; no-subject; digests; suspicious-header CC: Alyssa Ross , Bo Chen , Rob Bradford , Wei Liu , Sebastien Boeuf , qemu-devel@nongnu.org, dev@lists.cloudhypervisor.org, Spectrum OS Development , "Michael S. Tsirkin" , Stefano Garzarella , Alex =?iso-8859-1?Q?Benn=E9e?= , Manos Pitsidianakis , =?iso-8859-1?Q?Marc-Andr=E9?= Lureau , virtio-comment@lists.linux.dev X-Mailman-Version: 3.3.9 Precedence: list List-Id: Patches and low-level development discussion Archived-At: <> List-Archive: <> List-Help: List-Owner: List-Post: List-Subscribe: List-Unsubscribe: On Tue, Mar 03, 2026 at 12:56:46PM -0500, Demi Marie Obenour wrote: > Spectrum (https://spectrum-os.org) is going to be implementing > virtio devices outside of the host. One proposed method of doing > this is virtio-vhost-user, which is a virtio device that allows a > VM to expose a vhost-user device to another VM. For instance, one > could assign a NIC to one VM and have it provide a vhost-user-net > device for use by a different VM. > > I brought this up on the KVM/QEMU community call today. Alex Bennée > recommended using virtio-msg instead. However, I have a few concerns > with this: > > 1. Virtio-msg buses are specific to a given hypervisor or (in the > case of FF-A) to a given CPU architecture. None of the current > buses support KVM on platforms other than Arm64. Therefore, > a brand-new bus would be needed. Hi Demi, The virtio-msg AMP PCI bus works on KVM both for ARM and x86 and any other arch, it's generic. These are the patches we posted to qemu-devel: https://lore.kernel.org/qemu-devel/20260224155721.612314-1-edgar.iglesias@gmail.com/ > > 2. Virtio-msg requires not-yet-upstream drivers in both the frontend > (the VM using the device) and the backend (the VM providing the > device). Vhost-user uses any of the existing transports, such as > PCI or MMIO. This means that upstream drivers can be used in the > frontend, and also enables supports for Windows and other guests > that lack support for virtio-msg. Fair point, a bit of chicken and egg... > > 3. Vhost-user is already widely deployed, so frontend implementations > are quite well tested. A KVM-specific virtio-msg transport would > serve only one purpose: driver VMs (with assigned devices) on > non-Arm64 platforms. This is a quite niche use-case. Therefore, > I'm concerned that the needed frontend code will be poorly tested > and bitrot. > > Manos Pitsidianakis stated that vhost-user does not make sense in > this case. Why is that? Would it make sense to use virtio-msg > between VMM and its VM, and expose a vhost-user device to the > outside world? What about having the virtio-vhost-user guest driver > emulate a virtio-msg transport, so that it can be used with any device > implementation supporting virtio-msg? > > I would greatly appreciate any and all suggestions here. This is a > serious project that is going to be used in production, but I want > to ensure that the design is the best possible. I've not looked in detail at virtio-vhost-user but it seems to be a bit similar to virtio-msg over PCI AMP with some differences. My take is: vhost-user is an interface designed for splitting virtio transport from virtio device backend in user-space on the same OS instance. The virtio-vhost-user device, enables the use of vhost-user across VM's streching vhost-user's intended use. This is a great step but Since vhost-user is not a virtio transport, It is not end-to-end in the sense that you need QEMU to translate virtio-pci to vhost-user and tunnel it over the virtio-vhost-user device to the backend. Virtio-msg is from the start a transport meant to work between heterogenous systems, different OS, OS instances even different SoCs. It's end-to-end in the sense that if you have a front-end driver in the front-end VM with an appropriate virtio-msg bus you can talk directly with the backend without intermediate proxies, potentially without VM exits. Cheers, Edgar