cpu/x86/64bit/mode_switch2: The reverse function to mode_switch

Add another mode_switch assembly function to call x86_64 code from
x86_32 code. This is particullary useful for BLOBs like mrc.bin or
FSP that calls back into coreboot.

The user must first wrap all functions that are to be called from
x86_32 using the macro prot2lm_wrapper. Instead of using the original
function the wrapped functions must be passed to the x86_32 BLOBs.

The assembly code assume that 0-3 32bit arguments are passed to
the wrapped function.

Tested:
- Called x86_64 code from x86_32 code in qemu.
- Booted Lenovo X220 using x86_32 MRC using x86_64 console.

Change-Id: Ib625233e5f673eae9f3dcb2d03004c06bb07b149
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/79753
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

src/cpu/x86/64bit/Makefile.inc

@@ -1,6 +1,7 @@
 ## SPDX-License-Identifier: GPL-2.0-only
 
 all_x86-y += mode_switch.S
+all_x86-y += mode_switch2.S
 
 # Add --defsym=_start=0 to suppress a linker warning.
 $(objcbfs)/pt: $(dir)/pt.S $(obj)/config.h

src/cpu/x86/64bit/mode_switch2.S

@@ -0,0 +1,48 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Calls a x86_64 function from x86_32 context.
+ * Must not be directly invoked from C code!
+ */
+
+.text
+.code32
+	.section ".text.long_mode_call_3arg", "ax", @progbits
+	.global long_mode_call_3arg
+long_mode_call_3arg:
+
+	/* Function to call is passed in EAX. */
+
+	/* Backup registers */
+	pushal
+
+	/* Backup stack pointer */
+	mov	%esp, %ebp
+
+	/* Enter long mode, preserves ebx */
+	#include <cpu/x86/64bit/entry64.inc>
+
+	/* Align stack */
+	movabs	$0xfffffffffffffff0, %rax
+	andq	%rax, %rsp
+
+	movl	28(%rbp), %ebx	/* Function to call */
+	movl	36(%rbp), %edi	/* 1st arg */
+	movl	40(%rbp), %esi	/* 2nd arg */
+	movl	44(%rbp), %edx	/* 3rd arg */
+
+	call	*%rbx
+
+	/* Store return value on stack. popal will fetch it. */
+	mov	%eax, 28(%rbp)
+	shr	$32, %rax
+	movl	%eax, 24(%rbp)
+
+	#include <cpu/x86/64bit/exit32.inc>
+
+	/* Restore stack pointer */
+	mov	%ebp, %esp
+
+	/* Restore registers */
+	popal
+
+	ret

src/cpu/x86/64bit/prot2long.inc

@@ -0,0 +1,27 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+.text
+.code32
+/*
+ * Macro to wrap a x86_64 function to be called from x86_32 code.
+ * This assumes that 0-3 32bit arguments are passed to the
+ * calling function.
+ *
+ * In order to preserve ESP without setting up a stack frame
+ * pass the function to call in EAX. The macro prepends "__prot2lm_"
+ * to the wrapped function name.
+ */
+.macro prot2lm_wrapper func2call:req
+	.global __prot2lm_\func2call
+__prot2lm_\func2call :
+	/* Get function to call */
+	mov	$\func2call, %eax
+
+	/*
+	 * Jump to function instead of call.
+	 * It will return directly to caller.
+	 */
+	jmp	long_mode_call_3arg
+
+	/* Not reachable */
+.endm