Linux kernel module ABI (x86)

Question

I'm looking for an official definition of the ABI used for calls in kernel modules on x86. I recently looked into a compiled module and was surprised to discover that:

• it uses registers to pass the first three arguments (eax, edx and ecx).

• it's not fastcall as described in GCC docs. The docs say that fastcall uses ecx and edx (Microsoft style).

The only somewhat relevant info I could find was this, but it's not saying much at all.

So:

1) Is there any official documentation on the ABI used in kernel modules (not the syscall ABI and not just the function list - those I did find).

2) How is this weird fastcall produced by the compiler? Is there a switch I missed?

Answer 1

-mregparm=3 is responsible for this.