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

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Date:   Wed, 30 Mar 2022 08:58:20 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     Lu Baolu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        Christoph Hellwig <hch@...radead.org>,
        "Raj, Ashok" <ashok.raj@...el.com>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Eric Auger <eric.auger@...hat.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC v2 02/11] iommu: Add iommu_group_singleton_lockdown()

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?
 
> > 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.

Jason