Re: [RFC 0/7] Support in-kernel DMA with PASID and SVA

[Jason Gunthorpe <jgg@nvidia.com>]

On Sat, Oct 02, 2021 at 01:24:54AM +1300, Barry Song wrote:

> I assume KVA mode can avoid this iotlb flush as the device is using
> the page table of the kernel and sharing the whole kernel space. But
> will users be glad to accept this mode?

You can avoid the lock be identity mapping the physical address space
of the kernel and maping map/unmap a NOP.

KVA is just a different way to achive this identity map with slightly
different security properties than the normal way, but it doesn't
reach to the same security level as proper map/unmap.

I'm not sure anyone who cares about DMA security would see value in
the slight difference between KVA and a normal identity map.

> which have been mapped in the current dma-map/unmap with IOMMU backend.
> some drivers are using bouncing buffer to overcome the performance loss of
> dma_map/unmap as copying is faster than unmapping:
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=907676b130711fd1f

It is pretty unforuntate that drivers are hard coding behaviors based
on assumptions of what the portable API is doing under the covers.

Jason