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

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Date:   Wed, 30 Mar 2022 09:04:34 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     Jason Gunthorpe <jgg@...dia.com>,
        "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()

On Wed, 30 Mar 2022 14:18:47 +0000
"Tian, Kevin" <kevin.tian@...el.com> wrote:

> +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.

You only need to look so far as the dma alias quirks to find devices
that use the wrong RID for DMA.  In general I don't think we have
enough confidence to say that for all these devices the wrong RID is
exclusively used versus some combination of both RIDs.  Also, the
aliased RID is not always a physical device.  Thanks,

Alex