Sorry for the maybe not so helpful title.

Here is the problem :

I'm running Linux on a Mac pro 1,1 (the first x86 mac pro). It's a dual xeon 5150 with ECC ram. I have 2 ram kits in it : 2x512M and 2x2G (this one : http://www.ec.kingston.com/ecom/hyperx_us/partsinfo.asp?root=&ktcpartno=KTA-MP667AK2/4G)

If I only have the 2x512M kit everything works fine for all kernel versions but if I have both kits or just the 2x2G kit any kernel above 4.10 panics very early on (picture of said panic https://imgur.com/a/PipU5Oc). The picture was taken on 4.15 (using earlyprintk=efi,keep) on other versions even using earlyprintk I don't get any output.

I have been trying several kernels and everything up to 4.11 works no problem. Then on 4.11 I got a panic which mentionned NX and pages being in W+X which prompted me to try noexec=off on newer versions and that fixes the panic. This works up to 5.2.5.

/proc/cpuinfo reports that the CPU support the NX flag.

I would need help in order to troubleshoot this further.