MSI-X vector limits and reserving a virtio device ID

MSI-X vector limits and reserving a virtio device ID

[Demi Marie Obenour]

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I'd like to reserve a virtio device ID for virtio vhost-guest,
formally virtio vhost-user.  Would this be possible?

Also, I'd like to know which systems have a small limit on the number
of MSI-X vectors.  The vhost-guest device uses lots of MSI-X vectors.
Typical implementations will use 256 * 3 + 7 = 775 of them!  That can,
of course, be reduced by sharing vectors, but this is undesirable
as it comes at a performance cost.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)

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Re: MSI-X vector limits and reserving a virtio device ID

[Michael S. Tsirkin]

On Wed, May 27, 2026 at 07:53:11PM -0400, Demi Marie Obenour wrote:
> I'd like to reserve a virtio device ID for virtio vhost-guest,
> formally virtio vhost-user.  Would this be possible?

yes, follow the regular process for reserving IDs.

> Also, I'd like to know which systems have a small limit on the number
> of MSI-X vectors.  The vhost-guest device uses lots of MSI-X vectors.
> Typical implementations will use 256 * 3 + 7 = 775 of them!  That can,
> of course, be reduced by sharing vectors, but this is undesirable
> as it comes at a performance cost.

A single cpu x86 system?

> -- 
> Sincerely,
> Demi Marie Obenour (she/her/hers)

Re: MSI-X vector limits and reserving a virtio device ID

[Demi Marie Obenour]

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On 5/28/26 01:22, Michael S. Tsirkin wrote:
> On Wed, May 27, 2026 at 07:53:11PM -0400, Demi Marie Obenour wrote:
>> I'd like to reserve a virtio device ID for virtio vhost-guest,
>> formally virtio vhost-user.  Would this be possible?
> 
> yes, follow the regular process for reserving IDs.

Is there a process beyond just sending a message to the mailing list?
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)

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Re: MSI-X vector limits and reserving a virtio device ID

[Michael S. Tsirkin]

On Thu, May 28, 2026 at 03:05:18AM -0400, Demi Marie Obenour wrote:
> On 5/28/26 01:22, Michael S. Tsirkin wrote:
> > On Wed, May 27, 2026 at 07:53:11PM -0400, Demi Marie Obenour wrote:
> >> I'd like to reserve a virtio device ID for virtio vhost-guest,
> >> formally virtio vhost-user.  Would this be possible?
> > 
> > yes, follow the regular process for reserving IDs.
> 
> Is there a process beyond just sending a message to the mailing list?
> -- 
> Sincerely,
> Demi Marie Obenour (she/her/hers)

https://github.com/oasis-tcs/virtio-spec/blob/master/README.md#use-of-github-issues

However, there seem to be some opposition to creating this device.

-- 
MST

RE: MSI-X vector limits and reserving a virtio device ID

[Parav Pandit]

> 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.

> Also, I'd like to know which systems have a small limit on the number
> of MSI-X vectors.  
I didn’t follow the question. Typically, Intel cpu has a limit of 256 vectors per core.

> The vhost-guest device uses lots of MSI-X vectors.
> Typical implementations will use 256 * 3 + 7 = 775 of them!  That can,
> of course, be reduced by sharing vectors, but this is undesirable
> as it comes at a performance cost.
Did you get a chance to measure the performance, for which device type?
Interrupts are generally moderated.
In other devices (scalable functions of mlx5) I have measured perf of 8 to 16 queues sharing a vectors has < 4% perf drop.

> --
> Sincerely,
> Demi Marie Obenour (she/her/hers)

Re: MSI-X vector limits and reserving a virtio device ID

[Manos Pitsidianakis]

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.

>
> > Also, I'd like to know which systems have a small limit on the number
> > of MSI-X vectors.
> I didn’t follow the question. Typically, Intel cpu has a limit of 256 vectors per core.
>
> > The vhost-guest device uses lots of MSI-X vectors.
> > Typical implementations will use 256 * 3 + 7 = 775 of them!  That can,
> > of course, be reduced by sharing vectors, but this is undesirable
> > as it comes at a performance cost.
> Did you get a chance to measure the performance, for which device type?
> Interrupts are generally moderated.
> In other devices (scalable functions of mlx5) I have measured perf of 8 to 16 queues sharing a vectors has < 4% perf drop.
>
> > --
> > Sincerely,
> > Demi Marie Obenour (she/her/hers)

-- 
Manos Pitsidianakis
Emulation and Virtualization Engineer at Linaro Ltd

Vhost-guest (was virtio vhost-user) vs virtio-msg

[Demi Marie Obenour]

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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)

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Re: Vhost-guest (was virtio vhost-user) vs virtio-msg

[Alyssa Ross]

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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@gmail.com/
  https://lore.kernel.org/virtio-comment/89d13644-04ac-4057-b197-a6efe5e82ef6@gmail.com/

• Inquiry into the possibility of this functionality:
  https://lore.kernel.org/virtualization/ff2b81b0-04ba-49b7-8a4e-089f1c875744@suse.com/

• Most recent previous draft VIRTIO spec submission, with links to
  implementations:
  https://lore.kernel.org/virtio-dev/20221007165643.3920613-1-usama.arif@bytedance.com/raw

• Another inquiry about how to do it:
  https://lore.kernel.org/qemu-devel/CADx_CBPzAstC0o9X6CrnyFqYYAtPbw5-XHWxmXTt6+LyYb-U3g@mail.gmail.com/

• Implementation in crosvm:
  https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3146213

• 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-ndragazis@arrikto.com/

(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.

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Re: Vhost-guest (was virtio vhost-user) vs virtio-msg

[Stefan Hajnoczi]

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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@gmail.com/
>   https://lore.kernel.org/virtio-comment/89d13644-04ac-4057-b197-a6efe5e82ef6@gmail.com/
> 
> • Inquiry into the possibility of this functionality:
>   https://lore.kernel.org/virtualization/ff2b81b0-04ba-49b7-8a4e-089f1c875744@suse.com/
> 
> • Most recent previous draft VIRTIO spec submission, with links to
>   implementations:
>   https://lore.kernel.org/virtio-dev/20221007165643.3920613-1-usama.arif@bytedance.com/raw
> 
> • Another inquiry about how to do it:
>   https://lore.kernel.org/qemu-devel/CADx_CBPzAstC0o9X6CrnyFqYYAtPbw5-XHWxmXTt6+LyYb-U3g@mail.gmail.com/
> 
> • Implementation in crosvm:
>   https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3146213
> 
> • 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-ndragazis@arrikto.com/
> 
> (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

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Re: Vhost-guest (was virtio vhost-user) vs virtio-msg

[Alex Bennée]

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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

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Re: MSI-X vector limits and reserving a virtio device ID

[Stefan Hajnoczi]

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On Thu, May 28, 2026 at 08:47:25AM +0300, 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 think the "virtio vhost-guest" device is a continuation of the
virtio-vhost-user device work:
https://wiki.qemu.org/Features/VirtioVhostUser

It is a VIRTIO device type, not a transport.

The idea is that a driver VM runs a normal VIRTIO driver (e.g.
virtio-net-pci) and a second device VM runs the virtio-net device
implementation. It's similar to running a vhost-user-net device on the
host, except now the device implementation runs inside a VM instead of a
process on the host. The virtio-vhost-user device is what the device VM
sees and uses to provide the virtio-net device to the driver VM.

Stefan

> 
> >
> > > Also, I'd like to know which systems have a small limit on the number
> > > of MSI-X vectors.
> > I didn’t follow the question. Typically, Intel cpu has a limit of 256 vectors per core.
> >
> > > The vhost-guest device uses lots of MSI-X vectors.
> > > Typical implementations will use 256 * 3 + 7 = 775 of them!  That can,
> > > of course, be reduced by sharing vectors, but this is undesirable
> > > as it comes at a performance cost.
> > Did you get a chance to measure the performance, for which device type?
> > Interrupts are generally moderated.
> > In other devices (scalable functions of mlx5) I have measured perf of 8 to 16 queues sharing a vectors has < 4% perf drop.
> >
> > > --
> > > Sincerely,
> > > Demi Marie Obenour (she/her/hers)
> 
> -- 
> Manos Pitsidianakis
> Emulation and Virtualization Engineer at Linaro Ltd
> 

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Re: MSI-X vector limits and reserving a virtio device ID

[Demi Marie Obenour]

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On 5/28/26 01:22, Parav Pandit 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.

Virtio vhost-user is a device used to implement a vhost-user device
from another VM.

>> Also, I'd like to know which systems have a small limit on the number
>> of MSI-X vectors.  
> I didn’t follow the question. Typically, Intel cpu has a limit of 256 vectors per core.
> 
>> The vhost-guest device uses lots of MSI-X vectors.
>> Typical implementations will use 256 * 3 + 7 = 775 of them!  That can,
>> of course, be reduced by sharing vectors, but this is undesirable
>> as it comes at a performance cost.
> Did you get a chance to measure the performance, for which device type?
> Interrupts are generally moderated.
> In other devices (scalable functions of mlx5) I have measured perf of 8 to 16 queues sharing a vectors has < 4% perf drop.

I'm more concerned with whether it is a problem to assume that
775 MSI-X interrupts can be exposed by a virtual device.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)

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Re: MSI-X vector limits and reserving a virtio device ID

[Michael S. Tsirkin]

On Thu, May 28, 2026 at 03:03:00AM -0400, Demi Marie Obenour wrote:
> On 5/28/26 01:22, Parav Pandit 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.
> 
> Virtio vhost-user is a device used to implement a vhost-user device
> from another VM.
> 
> >> Also, I'd like to know which systems have a small limit on the number
> >> of MSI-X vectors.  
> > I didn’t follow the question. Typically, Intel cpu has a limit of 256 vectors per core.
> > 
> >> The vhost-guest device uses lots of MSI-X vectors.
> >> Typical implementations will use 256 * 3 + 7 = 775 of them!  That can,
> >> of course, be reduced by sharing vectors, but this is undesirable
> >> as it comes at a performance cost.
> > Did you get a chance to measure the performance, for which device type?
> > Interrupts are generally moderated.
> > In other devices (scalable functions of mlx5) I have measured perf of 8 to 16 queues sharing a vectors has < 4% perf drop.
> 
> I'm more concerned with whether it is a problem to assume that
> 775 MSI-X interrupts can be exposed by a virtual device.

Why not? But many hosts will have to share them.


> -- 
> Sincerely,
> Demi Marie Obenour (she/her/hers)