ARM assembly "retne" instruction

Question

I am currently in the process of understanding what it takes for the Linux kernel to boot. I was browsing through the Linux kernel source tree, in particular for the ARM architecture, until I stumbled upon this assembly instruction retne lr in arch/arm/kernel/hyp-stub.S

Conceptually, it's easily understood that the instruction is suppose to return to the address stored in the link register if the Z-flag is 0. What I am looking for is where this ARM assembly instruction is actually documented.

I searched in the ARM Architecture Reference Manual ARMv7-A and ARMv7-R edition section A8.8 and could not find the description of the instruction.

Grepping the sources and seeing if it was an ARM specific GNU AS extension did not turn up anything in particular.

A google search with the queries "arm assembly ret instruction", "arm return instruction" and anything similar along the lines did not turn up anything useful either. Surely I must be looking in the wrong places or I must be missing something.

Any clarification will be much appreciated.

Cedillo

Answer 1

The architectural assembly language is one thing, real world code is another. Once assembler pseudo-ops and macros come into play, a familiarity with both the toolchain and the codebase in question helps a lot. Linux is particularly nasty as much of the assembly source contains multiple layers of both assembler macros and CPP macros. If you know what to look for, and follow the header trail to arch/arm/include/asm/assembler.h, you eventually find this complicated beast:

.irp    c,,eq,ne,cs,cc,mi,pl,vs,vc,hi,ls,ge,lt,gt,le,hs,lo
.macro  ret\c, reg
#if __LINUX_ARM_ARCH__ < 6
        mov\c   pc, \reg
#else
        .ifeqs  "\reg", "lr"
        bx\c    \reg
        .else
        mov\c   pc, \reg
        .endif
#endif
        .endm
        .endr

The purpose of this is to emit the architecturally-preferred return instruction for the benefit of microarchitectures with a return stack, whilst allowing the same code to still compile for older architectures.

Footrest

See also the relevant commit message. – Marjorymarjy

No wonder why grepping the sources and seeing if it was an ARM specific GNU AS extension did not turn up anything either. Thank you, this clarifies it perfectly. – Cedillo