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I am trying to cross compile a device driver built for x86 architecture to arm platform. It got compiled without any errors, but I dont think whole features are available. So I checked the makefile and found this particular part.

ifeq ($(ARCH),x86_64)
    EXTRA_CFLAGS += -mcmodel=kernel -mno-red-zone

This is the only part that depends on architecture it seems. After some time on google, I found that -mcmodel=kernel is for kernel code model and -mno-red-zone is to avoid using red zone in memory and both them were for x86_64. But its not clear to me, what impact does it make setting cmodel to kernel?

(Any insight into the problem with arm is also greatly appreciated.)

fuz
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1 Answers1

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The x86 Options section of the GCC manual says:

-mcmodel=kernel

Generate code for the kernel code model. The kernel runs in the negative 2 GB of the address space.

(i.e. the upper 2GiB, addresses like 0xfffffffff0001234)

In the kernel code model, static symbol addresses don't fit in 32-bit zero-extended constants (unlike the default small code model where mov eax, imm32 (5 bytes) is the most efficient way to put a symbol address in a register).

But they do fit in sign-extended 32-bit constants, unlike the large code model for example. So mov rax, sign_extended_imm32 (7 bytes) works, and is the same size but maybe slightly more efficient than lea rax, [rel symbol].

But more importantly mov eax, [table + rdi*4] works, because disp32 displacements are sign-extended to 64 bits. -mcmodel=kernel tells gcc it can do this but not mov eax, table.

RIP-relative addressing can also reach any symbol from any code address (with a rel32 +-2GiB offset), so -fPIC or -fPIE will also make your code work, at the minor expense of not taking advantage of 32-bit absolute addressing in cases where it's useful. (e.g. indexing static arrays).

If you didn't get link errors without -mcmodel=kernel (like these), you probably have a gcc that makes PIE executables by default (common on recent distros), so it avoids absolute addressing.

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Peter Cordes
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