RE: [PATCH RFC v2 02/11] iommu: Add iommu_group_singleton_lockdown()

From: "Tian, Kevin" <kevin.tian@intel.com>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: "Alex Williamson \(alex.williamson@redhat.com\)"
	<alex.williamson@redhat.com>, "Raj, Ashok" <ashok.raj@intel.com>,
	Will Deacon <will@kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Christoph Hellwig <hch@infradead.org>,
	"iommu@lists.linux-foundation.org"
	<iommu@lists.linux-foundation.org>,
	"Pan, Jacob jun" <jacob.jun.pan@intel.com>,
	Jean-Philippe Brucker <jean-philippe@linaro.com>,
	Robin Murphy <robin.murphy@arm.com>
Subject: RE: [PATCH RFC v2 02/11] iommu: Add iommu_group_singleton_lockdown()
Date: Wed, 30 Mar 2022 14:18:47 +0000	[thread overview]
Message-ID: <BN9PR11MB52766A3E3BC82EEF437258198C1F9@BN9PR11MB5276.namprd11.prod.outlook.com> (raw)
In-Reply-To: 20220330115820.GE1716663@nvidia.com

+Alex

> From: Tian, Kevin
> Sent: Wednesday, March 30, 2022 10:13 PM
> 
> > From: Jason Gunthorpe
> > Sent: Wednesday, March 30, 2022 7:58 PM
> >
> > On Wed, Mar 30, 2022 at 06:50:11AM +0000, Tian, Kevin wrote:
> >
> > > One thing that I'm not very sure is about DMA alias. Even when physically
> > > there is only a single device within the group the aliasing could lead
> > > to multiple RIDs in the group making it non-singleton. But probably we
> > > don't need support SVA on such device until a real demand comes?
> >
> > How can we have multiple RIDs in the same group and have only one
> > device in the group?
> 
> Alex may help throw some insight here. Per what I read from the code
> looks like certain device can generate traffic with multiple RIDs.
> 
> >
> > > > ie if we have a singleton group that doesn't have ACS and someone
> > > > hotplugs in another device on a bridge, then our SVA is completely
> > > > broken and we get data corruption.
> > >
> > > Can we capture that in iommu_probe_device() when identifying
> > > the group which the probed device will be added to has already been
> > > locked down for SVA? i.e. make iommu_group_singleton_lockdown()
> > > in this patch to lock down the fact of singleton group instead of
> > > the fact of singleton driver...
> >
> > No, that is backwards
> >
> > > > Testing the group size is inherently the wrong test to make.
> > >
> > > What is your suggestion then?
> >
> > Add a flag to the group that positively indicates the group can never
> > have more than one member, even after hot plug. eg because it is
> > impossible due to ACS, or lack of bridges, and so on.
> >
> 
> OK, I see your point. It essentially refers to a singleton group which
> is immutable to hotplug.
> 
> Thanks
> Kevin
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