linux-kernel - RE: [RFC 13/20] iommu: Extend iommu_at[de]tach

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Message-ID: <BN9PR11MB54337A8E65C789D038D875C68CB89@BN9PR11MB5433.namprd11.prod.outlook.com>
Date:   Thu, 14 Oct 2021 07:06:07 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     David Gibson <david@...son.dropbear.id.au>,
        "Liu, Yi L" <yi.l.liu@...el.com>
CC:     "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "jgg@...dia.com" <jgg@...dia.com>, "hch@....de" <hch@....de>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "jean-philippe@...aro.org" <jean-philippe@...aro.org>,
        "parav@...lanox.com" <parav@...lanox.com>,
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        "corbet@....net" <corbet@....net>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "yi.l.liu@...ux.intel.com" <yi.l.liu@...ux.intel.com>,
        "Tian, Jun J" <jun.j.tian@...el.com>, "Wu, Hao" <hao.wu@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
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        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
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        "nicolinc@...dia.com" <nicolinc@...dia.com>
Subject: RE: [RFC 13/20] iommu: Extend iommu_at[de]tach_device() for multiple
 devices group

> From: David Gibson <david@...son.dropbear.id.au>
> Sent: Thursday, October 14, 2021 1:24 PM
> 
> On Sun, Sep 19, 2021 at 02:38:41PM +0800, Liu Yi L wrote:
> > From: Lu Baolu <baolu.lu@...ux.intel.com>
> >
> > These two helpers could be used when 1) the iommu group is singleton,
> > or 2) the upper layer has put the iommu group into the secure state by
> > calling iommu_device_init_user_dma().
> >
> > As we want the iommufd design to be a device-centric model, we want to
> > remove any group knowledge in iommufd. Given that we already have
> > iommu_at[de]tach_device() interface, we could extend it for iommufd
> > simply by doing below:
> >
> >  - first device in a group does group attach;
> >  - last device in a group does group detach.
> >
> > as long as the group has been put into the secure context.
> >
> > The commit <426a273834eae> ("iommu: Limit
> iommu_attach/detach_device to
> > device with their own group") deliberately restricts the two interfaces
> > to single-device group. To avoid the conflict with existing usages, we
> > keep this policy and put the new extension only when the group has been
> > marked for user_dma.
> 
> I still kind of hate this interface because it means an operation that
> appears to be explicitly on a single device has an implicit effect on
> other devices.
> 

I still didn't get your concern why it's such a big deal. With this proposal
the group restriction will be 'explicitly' documented in the attach uAPI
comment and sample flow in iommufd.rst. A sane user should read all
those information to understand how this new sub-system works and
follow whatever constraints claimed there. In the end the user should
maintain the same group knowledge regardless of whether to use an
explicit group uAPI or a device uAPI which has group constraint...

Thanks,
Kevin