linux-kernel - RE: [PATCH RFC v2 02/11] iommu: Add iommu_group_singleton

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Message-ID: <BN9PR11MB527691E38BAC4F89FB17BDB98C1F9@BN9PR11MB5276.namprd11.prod.outlook.com>
Date:   Wed, 30 Mar 2022 14:12:57 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     "Raj, Ashok" <ashok.raj@...el.com>, Will Deacon <will@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Christoph Hellwig" <hch@...radead.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        Robin Murphy <robin.murphy@....com>
Subject: RE: [PATCH RFC v2 02/11] iommu: Add iommu_group_singleton_lockdown()

> 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