sparc: fill in kernel entry code

- Fix elf32 vs elf64 (copied from EFI)
- Write the kernel start code

Change-Id: I6133185681307ecabc617b47b36551e52f9ac2c9
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3578
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

src/system/boot/platform/openfirmware/arch/sparc/arch_start_kernel.S

@@ -8,9 +8,40 @@
 /*	status_t arch_start_kernel(struct kernel_args *kernelArgs,
 		addr_t kernelEntry, addr_t kernelStackTop);
 
-	r3	- kernelArgs
-	r4	- kernelEntry
-	r5	- kernelStackTop
+	o0	- kernelArgs
+	o1	- kernelEntry
+	o2	- kernelStackTop
 */
 FUNCTION(arch_start_kernel):
-	return
+	save  %sp, -176, %sp
+
+	// Now we have
+	// - Kernel args in i0
+	// - Kernel entry in i1
+	// - Kernel stack top in i2
+
+	// Set the stack, adding the stack bias
+	sub %i2, 2047, %sp // apply stack bias
+
+	// We need to call the kernel with:
+	// o0 = kernel args
+	// o1 = current CPU
+	mov %i0, %o0
+	// Current CPU is always 0 on boot
+	mov %g0, %o1
+
+	// Jump into the kernel, store return address in o7
+	jmpl %i1, %o7
+	nop // branch delay slot...
+
+	// We should never get here, but in case the kernel returns, attempt to
+	// exit cleanly.
+
+	// Forward the return code to the caller
+	mov %o0, %i0
+
+	// Since we saved our context at the start of this function,
+	// a simple "return" should do the job and restore the previous register
+	// window and put us back on the old stack
+	return %i7 + 8
+	nop // Branch delay slot...

src/system/boot/platform/openfirmware/start.cpp

@@ -45,13 +45,27 @@ call_ctors(void)
 }
 
 
+static addr_t
+get_kernel_entry(void)
+{
+	if (gKernelArgs.kernel_image->elf_class == ELFCLASS64) {
+		preloaded_elf64_image *image = static_cast<preloaded_elf64_image *>(
+			gKernelArgs.kernel_image.Pointer());
+		return image->elf_header.e_entry;
+	} else if (gKernelArgs.kernel_image->elf_class == ELFCLASS32) {
+		preloaded_elf32_image *image = static_cast<preloaded_elf32_image *>(
+			gKernelArgs.kernel_image.Pointer());
+		return image->elf_header.e_entry;
+	}
+	panic("Unknown kernel format! Not 32-bit or 64-bit!");
+	return 0;
+}
+
+
 extern "C" void
 platform_start_kernel(void)
 {
-	preloaded_elf32_image* image = static_cast<preloaded_elf32_image*>(
-		gKernelArgs.kernel_image.Pointer());
-
-	addr_t kernelEntry = image->elf_header.e_entry;
+	addr_t kernelEntry = get_kernel_entry();
 	addr_t stackTop = gKernelArgs.cpu_kstack[0].start
 		+ gKernelArgs.cpu_kstack[0].size;