On 8/4/2016 2:56 PM, hch@infradead.org wrote:

quoted

Anyway, I understand exposing the CMB directly might be a little too
low level. What do you think would work better? Something like
direct_access that returns bus addresses instead of pfns?

This would be my primary choicse.  We'll really need to built an
in-kernel framework for a) PCI P2P access involving drivers for the PCI
switches etc, 

I was thinking perhaps the dma_map_resource patches by Will Davis [1]
could be used (adding him to the thread).

and then on top of that build something speicifly for
block drivers.  Which will be a bit complicated because both have the
case of directly memory mapped device as well as indirect devices like
the CMB.  I'm pretty sure we won't get it right at the first attempt,
so exposing it to userspace is an absolute no-go as that will lock us
into the first ABI forever.

I understand. I'll try to think of something for kernel usage only for
the next attempt.

Thanks,
Haggai

[1]
https://lists.linuxfoundation.org/pipermail/iommu/2015-August/014106.html