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

Jason Gunthorpe
2021-10-15T11:10:02Z

On Fri, Oct 15, 2021 at 01:29:16AM +0000, Tian, Kevin wrote:
> Hi, Jason,
>
> > From: Jason Gunthorpe <jgg@nvidia.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...

If in doubt make it

suppress_iommu_whatever_the_api_is_that_isn't_called

> 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.

Jason