Re: [RFC PATCH 4/4] hw/arm/smmuv3-accel: Read and propagate host vIOMMU events

[Zhangfei Gao <zhangfei.gao@linaro.org>]

On Fri, 14 Nov 2025 at 01:44, Nicolin Chen <nicolinc@nvidia.com> wrote:
>
> On Thu, Nov 13, 2025 at 05:07:50AM -0800, Shameer Kolothum wrote:
> > > On Wed, 5 Nov 2025 at 23:49, Shameer Kolothum
> > > <skolothumtho@nvidia.com> wrote:
> > > >
> > > > Install an event handler on the vEVENTQ fd to read and propagate host
> > > > generated vIOMMU events to the guest.
> > > >
> > > > The handler runs in QEMU’s main loop, using a non-blocking fd registered
> > > > via qemu_set_fd_handler().
> > > >
> > > > Signed-off-by: Shameer Kolothum <skolothumtho@nvidia.com>
> > >
> > > Still don't understand how to use this vevent.
> > > Is it to replace the fault queue (IOMMU_FAULT_QUEUE_ALLOC)?
> >
> > No. IIUC, IOMMU_FAULT_QUEUE_ALLOC is to handle I/O page faults
> > for any HWPT capable of handling page faults/response. The QEMU
> > SMMUv3 still don't support page fault handling.
> >
> > The VEVENTQ, on the other hand, provides a way to report any
> > other s1 events to Guest.
> >
> > See how events are reported in  arm_smmu_handle_event():
> >
> > if (event->stall)
> >     ret = iommu_report_device_fault(master->dev, &fault_evt); //Page faults
> > else if (master->vmaster && !event->s2)
> >     ret = arm_vmaster_report_event(master->vmaster, evt); //This series handles this case.
> > else
> >     ret = -EOPNOTSUPP;
>
> Yes. We can say that FAULT_QUEUE is exclusively for PRI while the
> vEVENTQ is for other types of HW events (or IRQs) related to the
> guest stage-1. They can be used together.
>
> > > And only find read, no write, only receive events but no response
> > > (from guest kernel)?
> >
> > Yes. And I am not sure what the long term plan is. We can still use
> > IOMMU_FAULT_QUEUE_ALLOC for page fault handling or extend this
> > VEVENTQ  to have write() support for responses
> >
> > To me, from an implementation perspective, both this FAULT and
> > VEVENTQ look almost similar.
> >
> > @Nicolin, any idea what's plan for page fault handling?
>
> No. I think PRI should be done via FAULT_QUEUE.

Does that mean FAULT_QUEUE needs a response, so read + write
VEVENTQ only notify, no need response, only read.

page faults need FAULT_QUEUE to resume.
dirty page record in userspace for live-migration etc, just use VEVENTQ?

>
> > > By the way, can we use vevent in user space application? not in qemu
> > > environment.
> >
> > I didn't get that. Qemu is userspace. Or you meant just to receive any events
> > from host SMMUv3 in user spacel?
>
> If user space application follows the iommufd uAPI like QEMU does,
> it can. I am not sure about the use case though.

Thanks Nico.

>
> Nicolin