Re: [PATCH v5 08/14] iommufd/viommu: Add iommufd_viommu_report_event helper

[Nicolin Chen]

On Fri, Jan 10, 2025 at 03:51:14PM -0400, Jason Gunthorpe wrote:
> On Fri, Jan 10, 2025 at 10:38:42AM -0800, Nicolin Chen wrote:
> > > The virtual event queue should behave the same as if the physical
> > > event queue overflows, and that logic should be in the smmu driver -
> > > this should return some Exxx to indicate the queue is filled.
> > 
> > Hmm, the driver only screams...
> > 
> > static irqreturn_t arm_smmu_evtq_thread(int irq, void *dev)
> > {
> > [...]
> > 		/*
> > 		 * Not much we can do on overflow, so scream and pretend we're
> > 		 * trying harder.
> > 		 */
> > 		if (queue_sync_prod_in(q) == -EOVERFLOW)
> > 			dev_err(smmu->dev, "EVTQ overflow detected -- events lost\n");
> 
> Well it must know from the HW somehow that the overflow has happened??
> 
> > > I supposed we will need a way to indicate lost events to userspace on
> > > top of this?
> > 
> > Perhaps another u32 flag in the arm_smmuv3_vevent struct to report
> > an overflow. That said, what userspace/VMM will need to do with it?
> 
> Trigger the above code in the VM somehow?

Oh, I see. I misunderstood somehow..

Thanks
Nicolin