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

[Tian, Kevin]

> From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Sent: Friday, October 15, 2021 7:10 PM
> 
> On Fri, Oct 15, 2021 at 01:29:16AM +0000, Tian, Kevin wrote:
> > 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...
> 
> If in doubt make it
> 
> suppress_iommu_whatever_the_api_is_that_isn't_called

ok

> 
> > 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.
> 
> It sounds reasonable, but also makes it much harder to find the few
> places that have this special relationship - ie we can't grep for
> DMA_OWNER_SHARED anymore.
> 

It's probably fine. People can just search the suppress flag and filter out
drivers with DMA_OWNER_USER. Then the remaining set is about 
drivers in SHARED category. Less straightforward but should be fine
for a relatively small set of drivers.

Thanks
Kevin