linux-kernel - RE: [RFC 06/20] iommu: Add iommu_device_init[exit]_user

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Message-ID: <BN9PR11MB5433C616093C8DDA5A859C2F8CB99@BN9PR11MB5433.namprd11.prod.outlook.com>
Date:   Fri, 15 Oct 2021 01:29:16 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     Lu Baolu <baolu.lu@...ux.intel.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "hch@....de" <hch@....de>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
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        "jean-philippe@...aro.org" <jean-philippe@...aro.org>,
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        "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>,
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        "jacob.jun.pan@...ux.intel.com" <jacob.jun.pan@...ux.intel.com>,
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Subject: RE: [RFC 06/20] iommu: Add iommu_device_init[exit]_user_dma
 interfaces

Hi, Jason,

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Wednesday, September 29, 2021 8:59 PM
> 
> On Wed, Sep 29, 2021 at 12:38:35AM +0000, Tian, Kevin wrote:
> 
> > /* If set the driver must call iommu_XX as the first action in probe() or
> >   * before it attempts to do DMA
> >   */
> >  bool suppress_dma_owner:1;
> 
> It is not "attempts to do DMA" but more "operates the physical device
> in any away"
> 
> Not having ownership means another entity could be using user space
> DMA to manipulate the device state and attack the integrity of the
> kernel's programming of the device.
> 

Does suppress_kernel_dma sounds better than suppress_dma_owner?
We found the latter causing some confusion when doing internal
code review. Somehow this flag represents "don't claim the kernel dma
ownership during driver binding". suppress_dma_owner sounds the
entire ownership is disabled...

Another thing is about DMA_OWNER_SHARED, which is set to indicate 
no dma at all. Thinking more we feel that this flag is meaningless. Its
sole purpose is to show compatibility to any USER/KERNEL ownership, 
and essentially the same semantics as a device which is not bound to
any driver. So we plan to remove it then pci-stub just needs one line 
change to set the suppress flag. But want to check with you first in case
any oversight.

Thanks
Kevin