linux-kernel - Re: Plan for /dev/ioasid RFC v2

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Date:   Thu, 24 Jun 2021 08:57:53 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     David Gibson <david@...son.dropbear.id.au>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        Joerg Roedel <joro@...tes.org>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Jason Wang <jasowang@...hat.com>,
        "parav@...lanox.com" <parav@...lanox.com>,
        "Enrico Weigelt, metux IT consult" <lkml@...ux.net>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Shenming Lu <lushenming@...wei.com>,
        Eric Auger <eric.auger@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>, "Wu, Hao" <hao.wu@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Kirti Wankhede <kwankhede@...dia.com>,
        Robin Murphy <robin.murphy@....com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        David Woodhouse <dwmw2@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>
Subject: Re: Plan for /dev/ioasid RFC v2

On Thu, Jun 24, 2021 at 02:37:31PM +1000, David Gibson wrote:
> On Thu, Jun 17, 2021 at 08:04:38PM -0300, Jason Gunthorpe wrote:
> > On Thu, Jun 17, 2021 at 03:02:33PM +1000, David Gibson wrote:
> > 
> > > In other words, do we really have use cases where we need to identify
> > > different devices IDs, even though we know they're not isolated.
> > 
> > I think when PASID is added in and all the complexity that brings, it
> > does become more important, yes.
> > 
> > At the minimum we should scope the complexity.
> > 
> > I'm not convinced it is so complicated, really it is just a single bit
> > of information toward userspace: 'all devices in this group must use
> > the same IOASID'
> 
> Um.. no?  You could have devA and devB sharing a RID, but then also
> sharing a group but not a RID with devC because of different isolation
> issues.  So you now have (at least) two levels of group structure to
> expose somehow.

Why? I don't need to micro optimize for broken systems. a/b/c can be
in the same group and the group can have the bit set. 

It is no worse than what we have today.

Jason