linux-kernel - Re: [PATCH v5 08/14] iommufd/viommu: Add iommufd_viommu_report

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Message-ID: <20250110195114.GJ5556@nvidia.com>
Date: Fri, 10 Jan 2025 15:51:14 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Nicolin Chen <nicolinc@...dia.com>
Cc: kevin.tian@...el.com, corbet@....net, will@...nel.org, joro@...tes.org,
	suravee.suthikulpanit@....com, robin.murphy@....com,
	dwmw2@...radead.org, baolu.lu@...ux.intel.com, shuah@...nel.org,
	linux-kernel@...r.kernel.org, iommu@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org,
	linux-kselftest@...r.kernel.org, linux-doc@...r.kernel.org,
	eric.auger@...hat.com, jean-philippe@...aro.org, mdf@...nel.org,
	mshavit@...gle.com, shameerali.kolothum.thodi@...wei.com,
	smostafa@...gle.com, ddutile@...hat.com, yi.l.liu@...el.com,
	patches@...ts.linux.dev
Subject: Re: [PATCH v5 08/14] iommufd/viommu: Add iommufd_viommu_report_event
 helper

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?

Jason