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

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Date:   Mon, 28 Jun 2021 13:26:04 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     "Alex Williamson (alex.williamson@...hat.com)" 
        <alex.williamson@...hat.com>, Joerg Roedel <joro@...tes.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        David Gibson <david@...son.dropbear.id.au>,
        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 Mon, Jun 28, 2021 at 06:45:23AM +0000, Tian, Kevin wrote:

> 7)   Unbinding detaches the device from the block_dma domain and
>        re-attach it to the default domain. From now on the user should 
>        be denied from accessing the device. vfio should tear down the
>        MMIO mapping at this point.

I think we should just forbid this, so long as the device_fd is open
the iommu_fd cannot be destroyed and there is no way to detact a
device other than closing its Fd.

revoke is tricky enough to implement we should avoid it.

> It's still an open whether we want to further allow devices within a group 
> attached to different IOASIDs in case that the source devices are reliably 
> identifiable. This is an usage not supported by existing vfio and might be
> not worthwhile due to improved isolation over time.

The main decision here is to decide if the uAPI should have some way to
indicate that a device does not have its own unique IOASID but is
sharing with the group

Jason