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

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Message-ID: <Z4F7fzLYtD/h9J4q@Asurada-Nvidia>
Date: Fri, 10 Jan 2025 11:56:47 -0800
From: Nicolin Chen <nicolinc@...dia.com>
To: Jason Gunthorpe <jgg@...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 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