On 5/28/26 01:47, Manos Pitsidianakis wrote:
On Thu, May 28, 2026 at 8:22 AM Parav Pandit <parav@nvidia.com> wrote:
From: Demi Marie Obenour <demiobenour@gmail.com> Sent: 28 May 2026 05:23 AM To: virtio-comment@lists.linux.dev Subject: MSI-X vector limits and reserving a virtio device ID
I'd like to reserve a virtio device ID for virtio vhost-guest, formally virtio vhost-user. Would this be possible?
Vhost user is an implementation of the device. I believe it stays as implementation and not a new device type.
This exactly.
Furthermore, we already have a mechanism for "providing" an arbitrary virtio device; it's called a transport.
Demi, I suggest you look into virtio-msg transport, which would allow you to do what you want.
I'm aware of virtio-msg, and in fact considered using it. However, I found that virtio-msg doesn't meet my requirements. 1. Virtio-msg needs to run over a transport of its own. None of the proposed transports support KVM guests on x86. FF-A is the only one that would make sense with KVM, but FF-A is specific to Arm. 2. Virtio-msg isn't compatible with existing frontend VMMs. Vhost-guest can be used with any frontend VMM that implements vhost-user. 3. Virtio-msg isn't compatible with existing frontend drivers. While I expect that drivers for Linux will eventually be upstreamed, I doubt that drivers for Windows or *BSD will ever be written. I don't even know if the Windows Driver Framework provides enough to write one. I don't need this myself, but I suspect that this is enough to make virtio-msg unsuitable in many environments. 4. Virtio-msg requires invasive changes to existing userspace device implementations. The project I work on doesn't use QEMU, and the existing frontends are targeted at server use-cases. Using the vhost-user protocol lets me reuse these with little effort. 5. To the best of my knowledge, virtio-msg doesn't support live migration of frontend VMs. Vhost-guest uses the vhost-user protocol, which has supported this for a very long time. I don't need live migration myself, but for many server use-cases, not having live migration is a dealbreaker. 6. I don't know if virtio-msg can achieve comparable performance. It appears to be optimized for reliability and isolation, not processing tens of gigabits of network traffic per second. Vhost-guest is designed with performance in mind. Vhost-guest is designed for storage and networking appliances on servers, whereas virtio-msg is designed for safety-critical embedded systems. These domains have very different requirements, and as a result they arrived at very different solutions. I work on Spectrum (https://spectrum-os.org), which uses Cloud Hypervisor. As the name implies, Cloud Hypervisor is primarily intended for cloud workloads, though it can also be used on clients. I don't think that an implementation of virtio-msg would be accepted, as it benefits none of Cloud Hypervisor's other users. -- Sincerely, Demi Marie Obenour (she/her/hers)
On Thu, May 28, 2026 at 02:59:13AM -0400, Demi Marie Obenour wrote:
On 5/28/26 01:47, Manos Pitsidianakis wrote:
On Thu, May 28, 2026 at 8:22 AM Parav Pandit <parav@nvidia.com> wrote:
From: Demi Marie Obenour <demiobenour@gmail.com> Sent: 28 May 2026 05:23 AM To: virtio-comment@lists.linux.dev Subject: MSI-X vector limits and reserving a virtio device ID
I'd like to reserve a virtio device ID for virtio vhost-guest, formally virtio vhost-user. Would this be possible?
Vhost user is an implementation of the device. I believe it stays as implementation and not a new device type.
This exactly.
Furthermore, we already have a mechanism for "providing" an arbitrary virtio device; it's called a transport.
Demi, I suggest you look into virtio-msg transport, which would allow you to do what you want.
I'm aware of virtio-msg, and in fact considered using it. However, I found that virtio-msg doesn't meet my requirements.
1. Virtio-msg needs to run over a transport of its own. None of the proposed transports support KVM guests on x86. FF-A is the only one that would make sense with KVM, but FF-A is specific to Arm.
2. Virtio-msg isn't compatible with existing frontend VMMs. Vhost-guest can be used with any frontend VMM that implements vhost-user.
3. Virtio-msg isn't compatible with existing frontend drivers. While I expect that drivers for Linux will eventually be upstreamed, I doubt that drivers for Windows or *BSD will ever be written. I don't even know if the Windows Driver Framework provides enough to write one. I don't need this myself, but I suspect that this is enough to make virtio-msg unsuitable in many environments.
4. Virtio-msg requires invasive changes to existing userspace device implementations. The project I work on doesn't use QEMU, and the existing frontends are targeted at server use-cases. Using the vhost-user protocol lets me reuse these with little effort.
5. To the best of my knowledge, virtio-msg doesn't support live migration of frontend VMs. Vhost-guest uses the vhost-user protocol, which has supported this for a very long time. I don't need live migration myself, but for many server use-cases, not having live migration is a dealbreaker.
6. I don't know if virtio-msg can achieve comparable performance. It appears to be optimized for reliability and isolation, not processing tens of gigabits of network traffic per second. Vhost-guest is designed with performance in mind.
Vhost-guest is designed for storage and networking appliances on servers, whereas virtio-msg is designed for safety-critical embedded systems. These domains have very different requirements, and as a result they arrived at very different solutions.
I work on Spectrum (https://spectrum-os.org), which uses Cloud Hypervisor. As the name implies, Cloud Hypervisor is primarily intended for cloud workloads, though it can also be used on clients. I don't think that an implementation of virtio-msg would be accepted, as it benefits none of Cloud Hypervisor's other users.
It would benefit them just as much as a virtio-vhost-user device would, surely? (And that's not a great justification from a spec point of view regardless…) But I think it's worth stepping back here and providing some context. virtio-vhost-user is an idea that's been kicking around in various forms since 2015, with incomplete implementations for QEMU, crosvm, and DPDK. It's been picked up by a number of different people during that time, and to my knowledge has only failed to make it across the line due to lack of investment. There's clear demand for a way to run a vhost-user device inside a VM. Links to previous iterations: • Discussions about VVU vs. virtio-msg: https://lore.kernel.org/virtio-comment/b291c8af-58ee-46e4-9033-e681ff9677db@... https://lore.kernel.org/virtio-comment/89d13644-04ac-4057-b197-a6efe5e82ef6@... • Inquiry into the possibility of this functionality: https://lore.kernel.org/virtualization/ff2b81b0-04ba-49b7-8a4e-089f1c875744@... • Most recent previous draft VIRTIO spec submission, with links to implementations: https://lore.kernel.org/virtio-dev/20221007165643.3920613-1-usama.arif@byted... • Another inquiry about how to do it: https://lore.kernel.org/qemu-devel/CADx_CBPzAstC0o9X6CrnyFqYYAtPbw5-XHWxmXTt... • Implementation in crosvm: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3146... • My own initial inquiry about it: https://lore.kernel.org/qemu-devel/87h7u1s5k1.fsf@alyssa.is/ • The last of another round of spec submissions: https://lore.kernel.org/virito-dev/1561418997-24608-1-git-send-email-ndragaz... (and more going back further) My point here is that VVU has been the go-to answer for how this problem could be solved for a very long time, during which an objection to a VIRTIO device used to implement other VIRTIO devices hasn't been widespread. I'd be interested to understand what's changed there, if indeed anything has. Is it just that virtio-msg now exists? Transporting VIRTIO over a stream isn't a huge leap, but the participants in prior discussions didn't suggest it as an alternative. From where I stand, virtio-msg seems at best an unproven and far-off way to accomplish this, whereas VVU has proven to work, at least well enough to have been considered worth putting in work to try to move the spec forward for multiple different (as far as I know) independent efforts over the years.
On Thu, May 28, 2026 at 10:56:35AM +0200, Alyssa Ross wrote:
On Thu, May 28, 2026 at 02:59:13AM -0400, Demi Marie Obenour wrote:
On 5/28/26 01:47, Manos Pitsidianakis wrote:
On Thu, May 28, 2026 at 8:22 AM Parav Pandit <parav@nvidia.com> wrote:
From: Demi Marie Obenour <demiobenour@gmail.com> Sent: 28 May 2026 05:23 AM To: virtio-comment@lists.linux.dev Subject: MSI-X vector limits and reserving a virtio device ID
I'd like to reserve a virtio device ID for virtio vhost-guest, formally virtio vhost-user. Would this be possible?
Vhost user is an implementation of the device. I believe it stays as implementation and not a new device type.
This exactly.
Furthermore, we already have a mechanism for "providing" an arbitrary virtio device; it's called a transport.
Demi, I suggest you look into virtio-msg transport, which would allow you to do what you want.
I'm aware of virtio-msg, and in fact considered using it. However, I found that virtio-msg doesn't meet my requirements.
1. Virtio-msg needs to run over a transport of its own. None of the proposed transports support KVM guests on x86. FF-A is the only one that would make sense with KVM, but FF-A is specific to Arm.
2. Virtio-msg isn't compatible with existing frontend VMMs. Vhost-guest can be used with any frontend VMM that implements vhost-user.
3. Virtio-msg isn't compatible with existing frontend drivers. While I expect that drivers for Linux will eventually be upstreamed, I doubt that drivers for Windows or *BSD will ever be written. I don't even know if the Windows Driver Framework provides enough to write one. I don't need this myself, but I suspect that this is enough to make virtio-msg unsuitable in many environments.
4. Virtio-msg requires invasive changes to existing userspace device implementations. The project I work on doesn't use QEMU, and the existing frontends are targeted at server use-cases. Using the vhost-user protocol lets me reuse these with little effort.
5. To the best of my knowledge, virtio-msg doesn't support live migration of frontend VMs. Vhost-guest uses the vhost-user protocol, which has supported this for a very long time. I don't need live migration myself, but for many server use-cases, not having live migration is a dealbreaker.
6. I don't know if virtio-msg can achieve comparable performance. It appears to be optimized for reliability and isolation, not processing tens of gigabits of network traffic per second. Vhost-guest is designed with performance in mind.
Vhost-guest is designed for storage and networking appliances on servers, whereas virtio-msg is designed for safety-critical embedded systems. These domains have very different requirements, and as a result they arrived at very different solutions.
I work on Spectrum (https://spectrum-os.org), which uses Cloud Hypervisor. As the name implies, Cloud Hypervisor is primarily intended for cloud workloads, though it can also be used on clients. I don't think that an implementation of virtio-msg would be accepted, as it benefits none of Cloud Hypervisor's other users.
It would benefit them just as much as a virtio-vhost-user device would, surely? (And that's not a great justification from a spec point of view regardless…)
But I think it's worth stepping back here and providing some context. virtio-vhost-user is an idea that's been kicking around in various forms since 2015, with incomplete implementations for QEMU, crosvm, and DPDK. It's been picked up by a number of different people during that time, and to my knowledge has only failed to make it across the line due to lack of investment. There's clear demand for a way to run a vhost-user device inside a VM.
Links to previous iterations:
• Discussions about VVU vs. virtio-msg: https://lore.kernel.org/virtio-comment/b291c8af-58ee-46e4-9033-e681ff9677db@... https://lore.kernel.org/virtio-comment/89d13644-04ac-4057-b197-a6efe5e82ef6@...
• Inquiry into the possibility of this functionality: https://lore.kernel.org/virtualization/ff2b81b0-04ba-49b7-8a4e-089f1c875744@...
• Most recent previous draft VIRTIO spec submission, with links to implementations: https://lore.kernel.org/virtio-dev/20221007165643.3920613-1-usama.arif@byted...
• Another inquiry about how to do it: https://lore.kernel.org/qemu-devel/CADx_CBPzAstC0o9X6CrnyFqYYAtPbw5-XHWxmXTt...
• Implementation in crosvm: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3146...
• My own initial inquiry about it: https://lore.kernel.org/qemu-devel/87h7u1s5k1.fsf@alyssa.is/
• The last of another round of spec submissions: https://lore.kernel.org/virito-dev/1561418997-24608-1-git-send-email-ndragaz...
(and more going back further)
My point here is that VVU has been the go-to answer for how this problem could be solved for a very long time, during which an objection to a VIRTIO device used to implement other VIRTIO devices hasn't been widespread. I'd be interested to understand what's changed there, if indeed anything has. Is it just that virtio-msg now exists? Transporting VIRTIO over a stream isn't a huge leap, but the participants in prior discussions didn't suggest it as an alternative. From where I stand, virtio-msg seems at best an unproven and far-off way to accomplish this, whereas VVU has proven to work, at least well enough to have been considered worth putting in work to try to move the spec forward for multiple different (as far as I know) independent efforts over the years.
Both virtio-vhost-user and virtio-msg their own pros/cons and use cases. Neither one comes out ahead in all cases, so I think it makes sense to have both. Demi, if you want to get the spec merged and are willing to participate in maintenance (answering questions, reviewing spec extensions, etc) then that would be nice. I will be around to transfer any knowledge I still have of virtio-vhost-user and review the patches. Stefan
Demi Marie Obenour <demiobenour@gmail.com> writes: (add virtio-msg authors to CC)
On 5/28/26 01:47, Manos Pitsidianakis wrote:
On Thu, May 28, 2026 at 8:22 AM Parav Pandit <parav@nvidia.com> wrote:
From: Demi Marie Obenour <demiobenour@gmail.com> Sent: 28 May 2026 05:23 AM To: virtio-comment@lists.linux.dev Subject: MSI-X vector limits and reserving a virtio device ID
I'd like to reserve a virtio device ID for virtio vhost-guest, formally virtio vhost-user. Would this be possible?
Vhost user is an implementation of the device. I believe it stays as implementation and not a new device type.
This exactly.
Furthermore, we already have a mechanism for "providing" an arbitrary virtio device; it's called a transport.
Demi, I suggest you look into virtio-msg transport, which would allow you to do what you want.
I'm aware of virtio-msg, and in fact considered using it. However, I found that virtio-msg doesn't meet my requirements.
1. Virtio-msg needs to run over a transport of its own. None of the proposed transports support KVM guests on x86. FF-A is the only one that would make sense with KVM, but FF-A is specific to Arm.
Well yes. virtio-msg is a common transport that implements various buses on the backend. FF-A is one but we have working implementations that just use plain memory sharing for the message bus which work on anything including x86/KVM. Although it does beg the question of what an additional transport would being to KVM as it is already well served by PCI and MMIO transports.
2. Virtio-msg isn't compatible with existing frontend VMMs. Vhost-guest can be used with any frontend VMM that implements vhost-user.
What exactly do you mean by frontend VMMs? The VMM needs some mechanism to expose a VirtIO transport to the guest be it through probing (PCI) or some machine description like ACPI or DT. The guest is completely unaware if the backend implementation is using vhost-user. If you were going to expose that to the guest you will need some mechanism for that. For what its worth there are QEMU and rust-vmm implementations of various virtio-msg backends although we would expect the host UAPI to be stabilised after the VirtIO spec is ratified (because its not guest visible).
3. Virtio-msg isn't compatible with existing frontend drivers. While I expect that drivers for Linux will eventually be upstreamed,
This is just plain wrong. No changes are needed to be made to the drivers as they are transport agnostic.
I doubt that drivers for Windows or *BSD will ever be written. I don't even know if the Windows Driver Framework provides enough to write one. I don't need this myself, but I suspect that this is enough to make virtio-msg unsuitable in many environments.
Why would Windows or *BSD want to implement virtio-msg when they can already use MMIO and PCI? But nothing stops them implementing it if they wish.
4. Virtio-msg requires invasive changes to existing userspace device implementations.
No it doesn't. We test virtio-msg with existing unmodified rust-vmm vhost-device implementations because on the host we bridge between virtio-msg and vhost-user. In QEMU the transport is abstracted away from the details of the device implementation - you don't need MMIO and PCI specific device implementations either.
The project I work on doesn't use QEMU, and the existing frontends are targeted at server use-cases. Using the vhost-user protocol lets me reuse these with little effort.
5. To the best of my knowledge, virtio-msg doesn't support live migration of frontend VMs. Vhost-guest uses the vhost-user protocol, which has supported this for a very long time. I don't need live migration myself, but for many server use-cases, not having live migration is a dealbreaker.
Live migration isn't in scope for a transport (aside from maybe support device reset/disable flows). The information needed to deal with migration is between the VMM and whatever implements the device backend. Indeed I find it a little confusing how live migration would work if the vhost-guest communication is directly between the backend and the guest. The VMM is the one that is responsible for serialistion and if it is cut out of the loop how will it know?
6. I don't know if virtio-msg can achieve comparable performance. It appears to be optimized for reliability and isolation, not processing tens of gigabits of network traffic per second. Vhost-guest is designed with performance in mind.
This is pure supposition. The data plane in virtio-msg is the same as in PCI and MMIO, shared memory and virtqs. While virtio-msg does support notifications in the message queue it does not preclude direct IRQ signalling or indeed switch to pure polling which is what most of the high speed networking solutions end up doing to avoid the latency of IRQs.
Vhost-guest is designed for storage and networking appliances on servers, whereas virtio-msg is designed for safety-critical embedded systems. These domains have very different requirements, and as a result they arrived at very different solutions.
I work on Spectrum (https://spectrum-os.org), which uses Cloud Hypervisor. As the name implies, Cloud Hypervisor is primarily intended for cloud workloads, though it can also be used on clients. I don't think that an implementation of virtio-msg would be accepted, as it benefits none of Cloud Hypervisor's other users.
-- Alex Bennée Virtualisation Tech Lead @ Linaro
participants (4)
-
Alex Bennée -
Alyssa Ross -
Demi Marie Obenour -
Stefan Hajnoczi