RE: [RFC 06/20] iommu: Add iommu_device_init[exit]_user_dma interfaces

[Tian, Kevin]

Hi, Jason,

> From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> 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