Re: [PATCH v7 3/5] hw/arm/smmuv3-accel: Allocate vEVENTQ for accelerated SMMUv3 devices

[Jonathan Cameron via <qemu-arm@nongnu.org>]

On Tue, 24 Feb 2026 14:41:08 +0000
Shameer Kolothum Thodi <skolothumtho@nvidia.com> wrote:

> > -----Original Message-----
> > From: Jonathan Cameron <jonathan.cameron@huawei.com>
> > Sent: 24 February 2026 14:29
> > To: Shameer Kolothum Thodi <skolothumtho@nvidia.com>
> > Cc: qemu-arm@nongnu.org; qemu-devel@nongnu.org;
> > eric.auger@redhat.com; peter.maydell@linaro.org; Nicolin Chen
> > <nicolinc@nvidia.com>; Nathan Chen <nathanc@nvidia.com>; Matt Ochs
> > <mochs@nvidia.com>; Jiandi An <jan@nvidia.com>; Jason Gunthorpe
> > <jgg@nvidia.com>; zhangfei.gao@linaro.org; zhenzhong.duan@intel.com;
> > Krishnakant Jaju <kjaju@nvidia.com>
> > 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 <skolothumtho@nvidia.com> wrote:
> >   
> > > From: Nicolin Chen <nicolinc@nvidia.com>
> > >
> > > 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.
Ok.  So maybe this is just a description thing that was confusing me.
The key to my mental model is that both conditions must be met. That
is SMMU_CR0 has EVENTQEN = 1 set and there is a VFIO device present.
They happen in different orders in the two cases?

What guarantees we don't get a hotplug event before the guest has
set EVENTQEN = 1?

Jonathan

> 
> Thanks,
> Shameer