Repost of my reddit post, but just wanted to save people the headache I ran into.
Asrock x370 boards have no issues with ECC with 16GB dimms and linux as long as
both banks A2 and B2 are populated with the same ram
The 2 dimm requirement is likely true for all memory dimm sizes.
Might be true for all am4 boards .
I don't know what is causing the issue currently ,could be a bios bug,kernel bug, or cpu limitation.
Save yourself the headache and just grab 2 sticks of ecc ram if you are going to be wanting to run in ECC mode on linux.
Working configuration
(tested on pro gaming : should work on k4 , haven't tested on my k4 yet, but should work without issue).
Distro: Ubuntu 17.04 beta
Kernel: 4.11 and 4.10 versions all worked.
Bios 1.60
x370 gaming pro board
2 Dimms banks A2 B2 with 16 GB each, total of 32 GB.
Memory used 2x KVR24E17D8/16MA (Kingston 2400 Micron memory version).
(Note I got it working with mixed configuration one KVR24E17D8/16MA and KVR24E17D8/16 also)
Samsung memory is on order, 16gb 2400mhz dimm. So I will know about if it also works with the asrock boards.
Should know if 64GB of ECC RAM will work soon, great for ZFS NAS
(Install hint if you get post error 46 , RAM is most likely not inserted far enough towards the bottom end of the ram.
Push down on the bottom, portion of the ram stick, part without the lock, clear the cmos and try again.)