linux-kernel - Re: [PATCH v7 13/14] iommu/arm-smmu-v3: Report events that belong to devices attached to vIOMMU

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Message-ID: <Z730M3XptvDRObBp@google.com>
Date: Tue, 25 Feb 2025 16:47:47 +0000
From: Pranjal Shrivastava <praan@...gle.com>
To: Nicolin Chen <nicolinc@...dia.com>
Cc: jgg@...dia.com, 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 v7 13/14] iommu/arm-smmu-v3: Report events that belong to
 devices attached to vIOMMU

On Mon, Feb 24, 2025 at 01:56:46PM -0800, Nicolin Chen wrote:
> On Mon, Feb 24, 2025 at 09:35:14PM +0000, Pranjal Shrivastava wrote:
> > On Sat, Feb 22, 2025 at 07:54:10AM -0800, Nicolin Chen wrote:
> > > +int arm_vmaster_report_event(struct arm_smmu_vmaster *vmaster, u64 *evt)
> > > +{
> > > +	struct iommu_vevent_arm_smmuv3 vevt;
> > > +	int i;
> > > +
> > > +	lockdep_assert_held(&vmaster->vsmmu->smmu->streams_mutex);
> > > +
> > > +	vevt.evt[0] = cpu_to_le64((evt[0] & ~EVTQ_0_SID) |
> > > +				  FIELD_PREP(EVTQ_0_SID, vmaster->vsid));
> > > +	for (i = 1; i < EVTQ_ENT_DWORDS; i++)
> > > +		vevt.evt[i] = cpu_to_le64(evt[i]);
> > 
> > Just thinking out loud here:
> > I understand the goal here is to "emulate" an IOMMU. But I'm just
> > wondering if we could report struct events instead of the raw event?
> > 
> > For example, can't we have something like arm_smmu_event here with the
> > sid changed to vsid? 
> > 
> > Are we taking the raw event since we want to keep the `u64 event_data[]`
> > field within `struct iommufd_vevent` generic to all architectures?
> 
> The ABIs for vSMMU are defined in the HW languange, e.g. cmd, ste.
> Thus, here evt in raw too.
> 

Ack. Makes sense.

> > > -	ret = iommu_report_device_fault(master->dev, &fault_evt);
> > > +	if (event->stall) {
> > > +		ret = iommu_report_device_fault(master->dev, &fault_evt);
> > > +	} else {
> > > +		if (master->vmaster && !event->s2)
> > > +			ret = arm_vmaster_report_event(master->vmaster, evt);
> > > +		else
> > > +			ret = -EFAULT; /* Unhandled events should be pinned */
> > > +	}
> > 
> > Nit:
> > I don't see the `arm_smmu_handle_event` being called elsewhere, is there
> > a reason to return -EFAULT instead of -EOPNOTSUPP here?
> > 
> > I think the current behavior here is to return -EOPNOTSUPP if (!event->stall).
> > Whereas, what we're doing here is:
> > 	if (event->stall) {
> > 	...
> > 	/* do legacy stuff */
> > 	...
> > 	}
> > 
> > 	else {
> > 		if (master->vmaster && !event->s2)
> > 			arm_vmaster_report_event(vmaster, evt);
> > 		else
> > 			ret = -EFAULT
> > 	}
> > 
> > 	mutex_unlock(&smmu->streams_mutex);
> > 	return ret;
> > 
> > Thus, we end up returning -EFAULT instead of -EOPNOTSUPP in case
> > event->stall == false. I agree that we aren't really checking the return
> > value in the evtq_thread handler, but I'm wondering if we should ensure
> > that we end up retaining the same behaviour as we have right now?
> 
> Oh, it looks like -EOPNOTSUPP should be returned here. Will fix.
> 

With the fix to return `-EOPNOTSUPP`:

Reviewed-by: Pranjal Shrivastava <praan@...gle.com>

Thanks,
Praan