arch/x86/boot: Jump to payload in protected mode

*   On ARCH_RAMSTAGE_X86_64 jump to the payload in protected mode.
*   Add a helper function to jump to arbitrary code in protected mode,
    similar to the real mode call handler.
*   Doesn't affect existing x86_32 code.
*   Add a macro to cast pointer to uint32_t that dies if it would overflow
    on conversion

Tested on QEMU Q35 using SeaBIOS as payload.
Tested on Lenovo T410 with additional x86_64 patches.

Change-Id: I6552ac30f1b6205e08e16d251328e01ce3fbfd14
Signed-off-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/30118
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

Documentation/arch/x86/index.md

@@ -15,6 +15,8 @@ In order to add support for x86_64 the following assumptions are made:
 * The high dword of pointers is always zero
 * The reference implementation is qemu
 * The CPU supports 1GiB hugepages
+* x86 payloads are loaded below 4GiB in physical memory and are jumped
+  to in *protected mode*
 
 ## Assuptions for all stages using the reference implementation
 * 0-4GiB are identity mapped using 2MiB-pages as WB
@@ -47,7 +49,7 @@ At the moment *$n* is 4, which results in identity mapping the lower 4 GiB.
 * Add assembly code for long mode - *DONE*
 * Add assembly code for SMM - *DONE*
 * Add assembly code for postcar stage - *DONE*
-* Add assembly code to return to protected mode - *TODO*
+* Add assembly code to return to protected mode - *DONE*
 * Implement reference code for mainboard `emulation/qemu-q35` - *TODO*
 
 ## Future work

src/arch/x86/Makefile.inc

@@ -245,6 +245,7 @@ ramstage-$(CONFIG_ACPI_BERT) += acpi_bert_storage.c
 ramstage-y += boot.c
 ramstage-y += post.c
 ramstage-y += c_start.S
+ramstage-y += c_exit.S
 ramstage-y += cpu.c
 ramstage-y += cpu_common.c
 ramstage-y += ebda.c

src/arch/x86/boot.c

@@ -1,10 +1,12 @@
 /* SPDX-License-Identifier: GPL-2.0-only */
 
+#include <arch/boot/boot.h>
 #include <commonlib/helpers.h>
 #include <console/console.h>
 #include <program_loading.h>
 #include <ip_checksum.h>
 #include <symbols.h>
+#include <assert.h>
 
 int payload_arch_usable_ram_quirk(uint64_t start, uint64_t size)
 {
@@ -19,6 +21,13 @@ int payload_arch_usable_ram_quirk(uint64_t start, uint64_t size)
 
 void arch_prog_run(struct prog *prog)
 {
+#if ENV_RAMSTAGE && defined(__x86_64__)
+	const uint32_t arg = pointer_to_uint32_safe(prog_entry_arg(prog));
+	const uint32_t entry = pointer_to_uint32_safe(prog_entry(prog));
+
+	/* On x86 coreboot payloads expect to be called in protected mode */
+	protected_mode_jump(entry, arg);
+#else
 #ifdef __x86_64__
 	void (*doit)(void *arg);
 #else
@@ -27,4 +36,5 @@ void arch_prog_run(struct prog *prog)
 #endif
 	doit = prog_entry(prog);
 	doit(prog_entry_arg(prog));
+#endif
 }

src/arch/x86/c_exit.S

@@ -0,0 +1,38 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#include <arch/ram_segs.h>
+#include <cpu/x86/msr.h>
+#include <cpu/x86/cr.h>
+
+
+#ifdef __x86_64__
+
+	/*
+	 * Functions to handle mode switches from long mode to protected
+	 * mode.
+	 */
+.text
+.code64
+	.section ".text.protected_mode_jump", "ax", @progbits
+	.globl protected_mode_jump
+protected_mode_jump:
+
+	push %rbp
+	mov %rsp, %rbp
+
+	/* Arguments to stack */
+	push	%rdi
+	push	%rsi
+
+	#include <cpu/x86/64bit/exit32.inc>
+
+	movl	-8(%ebp), %eax	/* Function to call */
+	movl	-16(%ebp), %ebx	/* Argument 0 */
+
+	/* Align the stack */
+	andl	$0xFFFFFFF0, %esp
+	subl	$12, %esp
+	pushl	%ebx	/* Argument 0 */
+
+	jmp	*%eax
+#endif

src/arch/x86/include/arch/boot/boot.h

@@ -7,4 +7,15 @@
 #define ELF_DATA	ELFDATA2LSB
 #define ELF_ARCH	EM_386
 
+#include <types.h>
+/*
+ * Jump to function in protected mode.
+ * @arg func_ptr           Function to jump to in protected mode
+ * @arg                    Argument to pass to called function
+ *
+ * @noreturn
+ */
+void protected_mode_jump(uint32_t func_ptr,
+			 uint32_t argument);
+
 #endif /* ASM_I386_BOOT_H */

src/include/assert.h

@@ -80,4 +80,15 @@ extern void _dead_code_assertion_failed(void) __attribute__((noreturn));
 	*(type *)(uintptr_t)0; \
 })
 
+#ifdef __x86_64__
+#define pointer_to_uint32_safe(x) ({ \
+	if ((uintptr_t)(x) > 0xffffffffUL) \
+		die("Cast from pointer to uint32_t overflows"); \
+	(uint32_t)(uintptr_t)(x); \
+})
+#else
+#define pointer_to_uint32_safe(x) ({ \
+	(uint32_t)(uintptr_t)(x); \
+})
+#endif
 #endif // __ASSERT_H__