On Tuesday, January 10, 2017 8:07:20 AM CET Christoph Hellwig wrote:

On Tue, Jan 10, 2017 at 09:47:21AM +0300, Nikita Yushchenko wrote:

quoted

I'm now working with HW that:
- is now way "low end" or "obsolete", it has 4G of RAM and 8 CPU cores,
and is being manufactured and developed,
- has 75% of it's RAM located beyond first 4G of address space,
- can't physically handle incoming PCIe transactions addressed to memory
beyond 4G.

It might not be low end or obselete, but it's absolutely braindead.
Your I/O performance will suffer badly for the life of the platform
because someone tries to save 2 cents, and there is not much we can do
about it.

Unfortunately it is a common problem for arm64 chips that were designed
by taking a 32-bit SoC and replacing the CPU core. The swiotlb is the
right workaround for this, and I think we all agree that we should
just make it work correctly.

	Arnd