> -----Original Message-----
> From: Jonathan Cameron <[email protected]>
> Sent: 24 February 2026 14:29
> To: Shameer Kolothum Thodi <[email protected]>
> Cc: [email protected]; [email protected];
> [email protected]; [email protected]; Nicolin Chen
> <[email protected]>; Nathan Chen <[email protected]>; Matt Ochs
> <[email protected]>; Jiandi An <[email protected]>; Jason Gunthorpe
> <[email protected]>; [email protected]; [email protected];
> Krishnakant Jaju <[email protected]>
> Subject: Re: [PATCH v7 3/5] hw/arm/smmuv3-accel: Allocate vEVENTQ for
> accelerated SMMUv3 devices
>
> External email: Use caution opening links or attachments
>
>
> On Thu, 19 Feb 2026 09:01:01 +0000
> Shameer Kolothum <[email protected]> wrote:
>
> > From: Nicolin Chen <[email protected]>
> >
> > When the guest enables the Event Queue and a vIOMMU is present,
> > allocate a vEVENTQ object so that host-side events related to the
> > vIOMMU can be received and propagated back to the guest.
> >
> > For cold-plugged devices using SMMUv3 acceleration, the vIOMMU is
> > created before the guest boots. In this case, the vEVENTQ is allocated
> > when the guest writes to SMMU_CR0 and sets EVENTQEN = 1.
> >
> > If no cold-plugged device exists at boot (i.e. no vIOMMU initially),
> > the vEVENTQ is allocated when a vIOMMU is created, i.e. during the
> > first device hot-plug.
>
> Why do it differently?
This is because a vEVENTQ must be associated with a vIOMMU, and
iommufd_backend_alloc_veventq() requires a valid viommu_id.
A vIOMMU is only created once at least one vfio-pci device is
attached. If the VM boots without any cold-plugged devices, no
vIOMMU exists at that point.
In that case, the vIOMMU is created during the first device
hot-plug, so we allocate the vEVENTQ at that time instead.
So, the difference in behaviour simply follows when the vIOMMU
becomes available.
Hope that is clear now.
Thanks,
Shameer
