Kernel: optimize yielding

Doing a yield no longer raises a software interrupt. Instead it just
saves all the callee saved registers, ip, sp and return value. Because
yield is only called in the kernel, it can just restore registers and
jump to the target address. There is never a need to use iret :)

kernel/arch/x86_64/Yield.S

@@ -0,0 +1,29 @@
+.global asm_yield_trampoline
+asm_yield_trampoline:
+	movq %rsp, %rcx
+	movq %rdi, %rsp
+
+	subq $8, %rsp
+	pushq (%rcx)
+	pushq %rcx
+	pushq %rax
+	pushq %rbx
+	pushq %rbp
+	pushq %r12
+	pushq %r13
+	pushq %r14
+	pushq %r15
+
+	movq %rsp, %rdi
+	call scheduler_on_yield
+
+	popq %r15
+	popq %r14
+	popq %r13
+	popq %r12
+	popq %rbp
+	popq %rbx
+	popq %rax
+	movq 8(%rsp), %rcx
+	movq 0(%rsp), %rsp
+	jmp *%rcx

kernel/arch/x86_64/interrupts.S

@@ -71,16 +71,6 @@ irq_stub:
 	addq $16, %rsp
 	iretq
 
-.global asm_yield_handler
-asm_yield_handler:
-	pushaq 8
-	cld
-	leaq 120(%rsp), %rdi # interrupt stack ptr
-	movq     %rsp,  %rsi # interrupt register ptr
-	call cpp_yield_handler
-	popaq 8
-	iretq
-
 .global asm_ipi_handler
 asm_ipi_handler:
 	pushaq 8