Macro hvc4 

macro_rules! hvc4 {
    ($num:expr, $arg0:expr, $arg1:expr, $arg2:expr, $arg3:expr) => { ... };
}

Generate an HVC call with 4 parameters

Puts the first argument in the instruction, and the parameters in r0-r3. Gives you back the value left in r0 by the handler.

const HYPERCALL_FOO: u32 = 0x100;
let result = hvc4!(0x00, HYPERCALL_FOO, 1, 2, 3);