efi: Support booting 32-bit kernels from 64-bit EFI

The default scheme for booting Linux kernels should be to switch to
32-bit protected mode and jump to the start of the kernel image. The
kernel has always had the know-how to switch 64-bit capable CPUs into
64-bit mode if necessary. By using this scheme, we can transparently
boot either 32-bit or 64-bit kernels.

This change necessitated moving kernel_jump() to a .S file for both i386
and x86-64. Writing inline assembly is fun for about 5 minutes, but then
becomes monstrously tedious.

Cc: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>

4 files changed, 74 insertions, 65 deletions

diff --git a/efi/Makefile b/efi/Makefile
index c89ca061..8e575ae2 100644
--- a/efi/Makefile
+++ b/efi/Makefile
@@ -38,7 +38,13 @@ LIB_OBJS = $(addprefix $(objdir)/com32/lib/,$(CORELIBOBJS))
 CSRC = $(wildcard $(SRC)/*.c)
 OBJS = $(subst $(SRC)/,,$(filter-out %wrapper.o, $(patsubst %.c,%.o,$(CSRC))))
 
-OBJS += $(objdir)/core/codepage.o
+OBJS += $(objdir)/core/codepage.o $(ARCH)/linux.o
+
+.PHONY: subdirs
+subdirs:
+	mkdir -p $(ARCH)
+
+$(OBJS): subdirs
 
 # The targets to build in this directory
 BTARGET  = syslinux.efi
diff --git a/efi/i386/linux.S b/efi/i386/linux.S
new file mode 100644
index 00000000..557d3e20
--- /dev/null
+++ b/efi/i386/linux.S
@@ -0,0 +1,20 @@
+/* ----------------------------------------------------------------------- *
+ *
+ *   Copyright 2013 Intel Corporation; author: Matt Fleming
+ *
+ *   This program is free software; you can redistribute it and/or modify
+ *   it under the terms of the GNU General Public License as published by
+ *   the Free Software Foundation, Inc., 51 Franklin St, Fifth Floor,
+ *   Boston MA 02110-1301, USA; either version 2 of the License, or
+ *   (at your option) any later version; incorporated herein by reference.
+ *
+ * ----------------------------------------------------------------------- */
+
+	.globl kernel_jump
+	.type  kernel_jump,@function
+	.text
+kernel_jump:
+	cli
+	movl	0x8(%esp), %esi
+	movl	0x4(%esp), %ecx
+	jmp	*%ecx
diff --git a/efi/main.c b/efi/main.c
index 71333a4b..0bae399b 100644
--- a/efi/main.c
+++ b/efi/main.c
@@ -417,74 +417,12 @@ struct boot_params {
  * allocate_pool()/free_pool()
  * memory_map()
  */ 
+extern void kernel_jump(EFI_PHYSICAL_ADDRESS kernel_start,
+			       struct boot_params *boot_params);
 #if __SIZEOF_POINTER__ == 4
 #define EFI_LOAD_SIG	"EL32"
-static inline void kernel_jump(EFI_PHYSICAL_ADDRESS kernel_start,
-			       struct boot_params *boot_params)
-{
-	asm volatile ("cli		\n"
-		      "movl %0, %%esi	\n"
-		      "movl %1, %%ecx	\n"
-		      "jmp *%%ecx	\n"
-		      :: "m" (boot_params), "m" (kernel_start));
-}
-
-static inline void handover_jump(EFI_HANDLE image, struct boot_params *bp,
-				 EFI_PHYSICAL_ADDRESS kernel_start)
-{
-	/* handover protocol not implemented yet; the linux header needs to be updated */
-#if 0
-	kernel_start += hdr->handover_offset;
-
-	asm volatile ("cli		\n"
-		      "pushl %0         \n"
-		      "pushl %1         \n"
-		      "pushl %2         \n"
-		      "movl %3, %%ecx	\n"
-		      "jmp *%%ecx	\n"
-		      :: "m" (bp), "m" (ST),
-		         "m" (image), "m" (kernel_start));
-#endif
-}
 #elif __SIZEOF_POINTER__ == 8
 #define EFI_LOAD_SIG	"EL64"
-typedef void(*kernel_func)(void *, struct boot_params *);
-typedef void(*handover_func)(void *, EFI_SYSTEM_TABLE *, struct boot_params *);
-static inline void kernel_jump(EFI_PHYSICAL_ADDRESS kernel_start,
-			       struct boot_params *boot_params)
-{
-	kernel_func kf;
-
-	asm volatile ("cli");
-
-	/* The 64-bit kernel entry is 512 bytes after the start. */
-	kf = (kernel_func)kernel_start + 512;
-
-	/*
-	 * The first parameter is a dummy because the kernel expects
-	 * boot_params in %[re]si.
-	 */
-	kf(NULL, boot_params);
-}
-
-static inline void handover_jump(EFI_HANDLE image, struct boot_params *bp,
-				 EFI_PHYSICAL_ADDRESS kernel_start)
-{
-#if 0
-	/* handover protocol not implemented yet the linux header needs to be updated */
-
-	UINT32 offset = bp->hdr.handover_offset;
-	handover_func hf;
-
-	asm volatile ("cli");
-
-	/* The 64-bit kernel entry is 512 bytes after the start. */
-	kernel_start += 512;
-
-	hf = (handover_func)(kernel_start + offset);
-	hf(image, ST, bp);
-#endif
-}
 #else
 #error "unsupported architecture"
 #endif
diff --git a/efi/x86_64/linux.S b/efi/x86_64/linux.S
new file mode 100644
index 00000000..4b1b88be
--- /dev/null
+++ b/efi/x86_64/linux.S
@@ -0,0 +1,45 @@
+/* ----------------------------------------------------------------------- *
+ *
+ *   Copyright 2013 Intel Corporation; author: Matt Fleming
+ *
+ *   This program is free software; you can redistribute it and/or modify
+ *   it under the terms of the GNU General Public License as published by
+ *   the Free Software Foundation, Inc., 51 Franklin St, Fifth Floor,
+ *   Boston MA 02110-1301, USA; either version 2 of the License, or
+ *   (at your option) any later version; incorporated herein by reference.
+ *
+ * ----------------------------------------------------------------------- */
+
+#define CR0_PG_FLAG	0x80000000
+#define MSR_EFER	0xc0000080
+
+	.globl kernel_jump
+	.type kernel_jump,@function
+	.code64
+kernel_jump:
+	cli
+
+	/*
+	 * Setup our segment selector (0x10) and return address (%rdi)
+	 * on the stack in preparation for the far return below.
+	 */
+	mov	$0x1000000000, %rcx
+	addq	%rcx, %rdi
+	pushq	%rdi
+
+	.code32
+pm_code:
+
+	/* Disable IA-32e mode by clearing IA32_EFER.LME */
+	xorl	%eax, %eax
+	xorl	%edx, %edx
+	movl	$MSR_EFER, %ecx
+	wrmsr
+
+	/* Turn off paging to disable long mode */
+	movl	%cr0, %eax
+	andl	$~CR0_PG_FLAG, %eax
+	movl	%eax, %cr0
+
+	/* Far return */
+	lret