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

On Thu, May 28, 2026 at 8:22=E2=80=AFAM Parav Pandit <[email protected]> wro=
te:
>
>
> > From: Demi Marie Obenour <[email protected]>
> > Sent: 28 May 2026 05:23 AM
> > To: [email protected]
> > 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=E2=80=99t 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 =3D 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 t=
o 16 queues sharing a vectors has < 4% perf drop.
>
> > --
> > Sincerely,
> > Demi Marie Obenour (she/her/hers)

--=20
Manos Pitsidianakis
Emulation and Virtualization Engineer at Linaro Ltd