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-rw-r--r--Documentation/x86/boot.txt38
-rw-r--r--arch/x86/boot/compressed/head_64.S22
2 files changed, 51 insertions, 9 deletions
diff --git a/Documentation/x86/boot.txt b/Documentation/x86/boot.txt
index 3edb4c2..0e38316 100644
--- a/Documentation/x86/boot.txt
+++ b/Documentation/x86/boot.txt
@@ -1054,6 +1054,44 @@ must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
must be __BOOT_DS; interrupt must be disabled; %esi must hold the base
address of the struct boot_params; %ebp, %edi and %ebx must be zero.
+**** 64-bit BOOT PROTOCOL
+
+For machine with 64bit cpus and 64bit kernel, we could use 64bit bootloader
+and we need a 64-bit boot protocol.
+
+In 64-bit boot protocol, the first step in loading a Linux kernel
+should be to setup the boot parameters (struct boot_params,
+traditionally known as "zero page"). The memory for struct boot_params
+could be allocated anywhere (even above 4G) and initialized to all zero.
+Then, the setup header at offset 0x01f1 of kernel image on should be
+loaded into struct boot_params and examined. The end of setup header
+can be calculated as follows:
+
+ 0x0202 + byte value at offset 0x0201
+
+In addition to read/modify/write the setup header of the struct
+boot_params as that of 16-bit boot protocol, the boot loader should
+also fill the additional fields of the struct boot_params as described
+in zero-page.txt.
+
+After setting up the struct boot_params, the boot loader can load
+64-bit kernel in the same way as that of 16-bit boot protocol, but
+kernel could be loaded above 4G.
+
+In 64-bit boot protocol, the kernel is started by jumping to the
+64-bit kernel entry point, which is the start address of loaded
+64-bit kernel plus 0x200.
+
+At entry, the CPU must be in 64-bit mode with paging enabled.
+The range with setup_header.init_size from start address of loaded
+kernel and zero page and command line buffer get ident mapping;
+a GDT must be loaded with the descriptors for selectors
+__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
+segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
+must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
+must be __BOOT_DS; interrupt must be disabled; %rsi must hold the base
+address of the struct boot_params.
+
**** EFI HANDOVER PROTOCOL
This protocol allows boot loaders to defer initialisation to the EFI
diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index 5c80b94..d9ae9a4 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -37,6 +37,12 @@
__HEAD
.code32
ENTRY(startup_32)
+ /*
+ * 32bit entry is 0 and it is ABI so immutable!
+ * If we come here directly from a bootloader,
+ * kernel(text+data+bss+brk) ramdisk, zero_page, command line
+ * all need to be under the 4G limit.
+ */
cld
/*
* Test KEEP_SEGMENTS flag to see if the bootloader is asking
@@ -182,20 +188,18 @@ ENTRY(startup_32)
lret
ENDPROC(startup_32)
- /*
- * Be careful here startup_64 needs to be at a predictable
- * address so I can export it in an ELF header. Bootloaders
- * should look at the ELF header to find this address, as
- * it may change in the future.
- */
.code64
.org 0x200
ENTRY(startup_64)
/*
+ * 64bit entry is 0x200 and it is ABI so immutable!
* We come here either from startup_32 or directly from a
- * 64bit bootloader. If we come here from a bootloader we depend on
- * an identity mapped page table being provied that maps our
- * entire text+data+bss and hopefully all of memory.
+ * 64bit bootloader.
+ * If we come here from a bootloader, kernel(text+data+bss+brk),
+ * ramdisk, zero_page, command line could be above 4G.
+ * We depend on an identity mapped page table being provided
+ * that maps our entire kernel(text+data+bss+brk), zero page
+ * and command line.
*/
#ifdef CONFIG_EFI_STUB
/*
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