Re: [RFC 4/5] PCI: vmd: Stop overriding dma_map_ops

["Derrick, Jonathan" <jonathan.derrick@intel.com>]

On Thu, 2020-01-09 at 15:36 +0100, Christoph Hellwig wrote:
> On Tue, Dec 31, 2019 at 01:24:22PM -0700, Jon Derrick wrote:
> > Devices on the VMD domain use the VMD endpoint's requester-id and have
> > been relying on the VMD endpoint's dma operations. The downside of this
> > was that VMD domain devices would use the VMD endpoint's attributes when
> > doing DMA and IOMMU mapping. We can be smarter about this by only using
> > the VMD endpoint when mapping and providing the correct child device's
> > attributes during dma operations.
> > 
> > This patch adds a new dma alias mechanism by adding a hint to a pci_dev
> > to point to a singular DMA requester's pci_dev. This integrates into the
> > existing dma alias infrastructure to reduce the impact of the changes
> > required to support this mode.
> 
> If we want to lift this check into common code I think it should go
> into struct device, as that is what DMA operates on normally.
I thought about that too, but the dma alias mechanism was in pci_dev. I
can prepare a new version with struct device.

>   That
> being said given that this insane hack only exists for braindamage in
> Intel hardware I'd rather keep it as isolated as possible. 
jmho but the footprint of the new set is pretty minimal and removes a
lot of dubious code in vmd.c.