[ndesaulniers]

Is it ever the case that inline assembly is required over separate object sources just in assembly? I would have thought it would be preferred to not use inline assembly, and simply link in object files of what you need. It would seem simpler syntax-wise, too. Why prefer inline assembly?

[cyphar]

There's several macros in the kernel which contain inline assembly, and you can't use code written in assembly because it would require using the stack to call the function (the case I'm thinking of is the switch_to macro which switches between tasks in the kernel).

[rwmj]

One reason is that you just want to call a single instruction, and the overhead of making a function call to another file would be too much. Picking an example from the Linux kernel pretty much at random:

    #define mb()    asm volatile("mfence":::"memory")
    #define rmb()   asm volatile("lfence":::"memory")
    #define wmb()   asm volatile("sfence" ::: "memory")

Those are memory barriers, so they essentially must be inline (they'd be too slow and possibly even change their meaning if they were located in a separate source file and you had to call them).