linux-kernel - Re: [PATCH v6 13/14] iommu/arm-smmu-v3: Report events that belong to devices attached to vIOMMU

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Message-ID: <Z7TZP3jXlRzweFE8@nvidia.com>
Date: Tue, 18 Feb 2025 11:02:23 -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 v6 13/14] iommu/arm-smmu-v3: Report events that belong to
 devices attached to vIOMMU

On Tue, Feb 18, 2025 at 02:50:46PM -0400, Jason Gunthorpe wrote:
> On Tue, Feb 18, 2025 at 10:28:04AM -0800, Nicolin Chen wrote:
> > On Tue, Feb 18, 2025 at 01:18:21PM -0400, Jason Gunthorpe wrote:
> > > On Fri, Jan 24, 2025 at 04:30:42PM -0800, Nicolin Chen wrote:
> > > 
> > > > @@ -1831,31 +1831,30 @@ static int arm_smmu_handle_event(struct arm_smmu_device *smmu,
> > > >  		return -EOPNOTSUPP;
> > > >  	}
> > > 
> > > There is still the filter at the top:
> > > 
> > > 	switch (event->id) {
> > > 	case EVT_ID_TRANSLATION_FAULT:
> > > 	case EVT_ID_ADDR_SIZE_FAULT:
> > > 	case EVT_ID_ACCESS_FAULT:
> > > 	case EVT_ID_PERMISSION_FAULT:
> > > 		break;
> > > 	default:
> > > 		return -EOPNOTSUPP;
> > > 	}
> > > 
> > > Is that right here or should more event types be forwarded to the
> > > guest?
> > 
> > That doesn't seem to be right. Something like EVT_ID_BAD_CD_CONFIG
> > should be forwarded too. I will go through the list.
> 
> I think the above should decode into a 'faultable' path because they
> all decode to something with an IOVA
> 
> The rest should decode to things that include a SID and the SID decode
> should always be forwarded to the VM. Maybe there are small
> exclusions, but generally that is how I would see it..

Ack. SMMU spec defines three type:
"Three categories of events might be recorded into the Event queue:
 • Configuration errors.
 • Faults from the translation process.
 • Miscellaneous."

The driver cares the first two only, as you remarked here.

> > > This already holds the streams_mutex across all of this, do you think
> > > we should get rid of the vmaster_rwsem and hold the streams_mutex on
> > > write instead?
> > 
> > They are per master v.s. per smmu. The latter one would make master
> > commits/attaches exclusive, which feels unnecessary to me, although
> > it would make the code here slightly cleaner..
> 
> I'd pay the cost on the attach side to have a single lock on the fault
> side..

OK. Maybe a small patch to turn the streams_mutex to streams_rwsem?

Thanks
Nicolin