It is the fourth part of the Kernel booting process and we will see first steps in the protected mode, like checking that cpu supports the long mode and SSE, paging and initialization of the page tables and transition to the long mode in in the end of this part.
NOTE: will be much assembly code in this part, so if you have poor knowledge, read a book about it
In the previous part we stopped at the jump to the 32-bit entry point in the arch/x86/boot/pmjump.S:
jmpl *%eaxRecall that eax register contains the address of the 32-bit entry point. We can read about this point from the linux kernel x86 boot protocol:
When using bzImage, the protected-mode kernel was relocated to 0x100000
And now we can make sure that it is true. Let's look on registers value in 32-bit entry point:
eax 0x100000 1048576
ecx 0x0 0
edx 0x0 0
ebx 0x0 0
esp 0x1ff5c 0x1ff5c
ebp 0x0 0x0
esi 0x14470 83056
edi 0x0 0
eip 0x100000 0x100000
eflags 0x46 [ PF ZF ]
cs 0x10 16
ss 0x18 24
ds 0x18 24
es 0x18 24
fs 0x18 24
gs 0x18 24
We can see here that cs register contains - 0x10 (as you can remember from the previous part, it is the second index in the Global Descriptor Table), eip register is 0x100000 and base address of the all segments include code segment is zero. So we can get physical address, it will be 0:0x100000 or just 0x100000, as in boot protocol. Now let's start with 32-bit entry point.
We can find the definition of the 32-bit entry point in arch/x86/boot/compressed/head_64.S:
__HEAD
.code32
ENTRY(startup_32)
....
....
....
ENDPROC(startup_32)First of all why compressed directory? Actually bzimage is a gzipped vmlinux + header + kernel setup code. We saw the kernel setup code in all of the previous parts. So, the main goal of the head_64.S is to prepare for entering long mode, enter into it and decompress the kernel. We will see all of these steps besides kernel decompression in this part.
Also you can note that there are two files in the arch/x86/boot/compressed directory:
- head_32.S
- head_64.S
We will see only head_64.S because we are learning linux kernel for x86_64. head_32.S even not compiled in our case. Let's look at arch/x86/boot/compressed/Makefile. We can see there the following target:
vmlinux-objs-y := $(obj)/vmlinux.lds $(obj)/head_$(BITS).o $(obj)/misc.o \
$(obj)/string.o $(obj)/cmdline.o \
$(obj)/piggy.o $(obj)/cpuflags.oNote on $(obj)/head_$(BITS).o. It means that compilation of the head_{32,64}.o depends on value of the $(BITS). We can find it in the other Makefile - arch/x86/kernel/Makefile:
ifeq ($(CONFIG_X86_32),y)
BITS := 32
...
...
else
...
...
BITS := 64
endifNow we know where to start, so let's do it.
As I wrote above, we start in arch/x86/boot/compressed/head_64.S. First of all we can see before the startup_32 definition:
__HEAD
.code32
ENTRY(startup_32)__HEAD is defined in include/linux/init.h and looks like:
#define __HEAD .section ".head.text","ax"We can find this section in the arch/x86/boot/compressed/vmlinux.lds.S linker script:
SECTIONS
{
. = 0;
.head.text : {
_head = . ;
HEAD_TEXT
_ehead = . ;
}
Note on . = 0;. . is a special variable of linker - location counter. The value assigned to it is an offset relative to the offset of the segment. As we assign zero to it, we can read from comments:
Be careful parts of head_64.S assume startup_32 is at address 0.
Ok, now we know where we are, and now is the best time to look inside the startup_32 function.
In the start of startup_32 we can see the cld instruction which clears the DF flag. After this, string operations like stosb and others will increment the index registers esi or edi.
Next we can see the check of the KEEP_SEGMENTS flag from loadflags. If you remember we already saw loadflags in the arch/x86/boot/head.S (there we checked flag CAN_USE_HEAP). Now we need to check the KEEP_SEGMENTS flag. We can find a description of this flag in the linux boot protocol:
Bit 6 (write): KEEP_SEGMENTS
Protocol: 2.07+
- If 0, reload the segment registers in the 32bit entry point.
- If 1, do not reload the segment registers in the 32bit entry point.
Assume that %cs %ds %ss %es are all set to flat segments with
a base of 0 (or the equivalent for their environment).
and if KEEP_SEGMENTS is not set, we need to set ds, ss and es registers to a flat segment with base 0. That we do:
testb $(1 << 6), BP_loadflags(%esi)
jnz 1f
cli
movl $(__BOOT_DS), %eax
movl %eax, %ds
movl %eax, %es
movl %eax, %ssremember that __BOOT_DS is 0x18 (index of data segment in the Global Descriptor Table). If KEEP_SEGMENTS is set, we jump to the label 1f or update segment registers with __BOOT_DS if this flag is not set.
If you read the previous part, you can remember that we already updated segment registers in the arch/x86/boot/pmjump.S, so why do we need to set up it again? Actually linux kernel also has the 32-bit boot protocol, so startup_32 can be the first function which will be executed right after a bootloader transfers control to the kernel.
As we checked the KEEP_SEGMENTS flag and put the correct value to the segment registers, the next step is to calculate difference between where we loaded and compiled to run (remember that setup.ld.S contains . = 0 at the start of the section):
leal (BP_scratch+4)(%esi), %esp
call 1f
1: popl %ebp
subl $1b, %ebpHere the esi register contains the address of the boot_params structure. boot_params contains a special field scratch with offset 0x1e4. We are getting the address of the scratch field + 4 bytes and puting it in the esp register (we will use it as stack for these calculations). After this we can see the call instruction and 1f label as its operand. What does call mean? It means that it pushes the ebp value into the stack, then the esp value, then the function arguments and returns the address in the end. After this we pop return address from the stack into ebp register (ebp will contain return address) and subtract address of the previous label 1.
After this we have address where we loaded in the ebp - 0x100000.
Now we can setup the stack and verify that the CPU supports long mode and SSE.
Next we can see assembly code which sets up a new stack for kernel decompression:
movl $boot_stack_end, %eax
addl %ebp, %eax
movl %eax, %espboots_stack_end is in the .bss section. We can see the definition of it in the end of head_64.S:
.bss
.balign 4
boot_heap:
.fill BOOT_HEAP_SIZE, 1, 0
boot_stack:
.fill BOOT_STACK_SIZE, 1, 0
boot_stack_end:First of all we put the address of boot_stack_end into the eax register and add to it value of ebp (remember that ebp now contains the address where we loaded - 0x100000). In the end we just put the eax value into esp and that's all, we have a correct stack pointer.
The next step is CPU verification. We need to check that the CPU supports long mode and SSE:
call verify_cpu
testl %eax, %eax
jnz no_longmodeIt just calls verify_cpu function from arch/x86/kernel/verify_cpu.S which contains a couple of calls to the cpuid instruction. cpuid is the instruction which is used for getting information about the processor. In our case it checks long mode and SSE support and returns 0 on success or 1 on fail in the eax register.
If eax is not zero, we jump to the no_longmode label which just stops the CPU with a hlt instruction while any hardware interrupt will not happen.
no_longmode:
1:
hlt
jmp 1bWe set stack, checked CPU and now can move on to the next step.
The next step is calculating relocation address for decompression if needed. We can see the following assembly code:
#ifdef CONFIG_RELOCATABLE
movl %ebp, %ebx
movl BP_kernel_alignment(%esi), %eax
decl %eax
addl %eax, %ebx
notl %eax
andl %eax, %ebx
cmpl $LOAD_PHYSICAL_ADDR, %ebx
jge 1f
#endif
movl $LOAD_PHYSICAL_ADDR, %ebx
1:
addl $z_extract_offset, %ebxFirst of all note on CONFIG_RELOCATABLE macro. This configuration option is defined in the arch/x86/Kconfig and as we can read from it's description:
This builds a kernel image that retains relocation information
so it can be loaded someplace besides the default 1MB.
Note: If CONFIG_RELOCATABLE=y, then the kernel runs from the address
it has been loaded at and the compile time physical address
(CONFIG_PHYSICAL_START) is used as the minimum location.
In short words, this code calculates the address to move the kernel to for decompression put it into the ebx register if the kernel is relocatable or bzimage will decompress itself above LOAD_PHYSICAL_ADDR.
Let's look at the code. If we have CONFIG_RELOCATABLE=n in our kernel configuration file, it just puts LOAD_PHYSICAL_ADDR into the ebx register and adds z_extract_offset to ebx. As ebx is zero for now, it will contain z_extract_offset. Now let's try to understand these two values.
LOAD_PHYSICAL_ADDR is the macro which defined in arch/x86/include/asm/boot.h and it looks like this:
#define LOAD_PHYSICAL_ADDR ((CONFIG_PHYSICAL_START \
+ (CONFIG_PHYSICAL_ALIGN - 1)) \
& ~(CONFIG_PHYSICAL_ALIGN - 1))Here we calculate the aligned address where the kernel is loaded (0x100000 or 1 megabyte in our case). PHYSICAL_ALIGN is an alignment value to which the kernel should be aligned, and it ranges from 0x200000 to 0x1000000 for x86_64. With the default values we will get 2 megabytes in the LOAD_PHYSICAL_ADDR:
>>> 0x100000 + (0x200000 - 1) & ~(0x200000 - 1)
2097152After we retrieve the alignment unit, we add z_extract_offset (which is 0xe5c000 in my case) to the 2 megabytes. In the end we will get 17154048 bytes offset. You can find z_extract_offset in arch/x86/boot/compressed/piggy.S. This file is generated in compile time by the mkpiggy program.
Now let's try to understand the code if CONFIG_RELOCATABLE is y.
First of all we put the ebp value into ebx (remember that ebp contains address where we loaded) and kernel_alignment field from kernel setup header into the eax register. kernel_alignment is a physical address of alignment required for the kernel. Next we do the same as in the previous case (when kernel is not relocatable), but we just use the value of the kernel_alignment field as the align unit and ebx (address where we loaded) as the base address instead of CONFIG_PHYSICAL_ALIGN and LOAD_PHYSICAL_ADDR.
After we calculate the address, we compare it with LOAD_PHYSICAL_ADDR and add z_extract_offset to it again or put LOAD_PHYSICAL_ADDR in the ebx if the calculated address is less than we need.
After all of this calculation we will have ebp which contains the address where we loaded and ebx with the address to which the kernel will be moved for decompression.
Now we need to do the last preparations before we can see the transition to 64-bit mode. At first we need to update the Global Descriptor Table for this:
leal gdt(%ebp), %eax
movl %eax, gdt+2(%ebp)
lgdt gdt(%ebp)Here we put the address from ebp with gdt offset into the eax register, next we put this address into ebp with offset gdt+2 and load the Global Descriptor Table with the lgdt instruction.
Let's look at the Global Descriptor Table definition:
.data
gdt:
.word gdt_end - gdt
.long gdt
.word 0
.quad 0x0000000000000000 /* NULL descriptor */
.quad 0x00af9a000000ffff /* __KERNEL_CS */
.quad 0x00cf92000000ffff /* __KERNEL_DS */
.quad 0x0080890000000000 /* TS descriptor */
.quad 0x0000000000000000 /* TS continued */It is defined in the same file as the .data section. It contains 5 descriptors: null descriptor, for kernel code segment, kernel data segment and two task descriptors. We already loaded the GDT in the previous part, we're doing almost the same here, but descriptors with CS.L = 1 and CS.D = 0 for execution in 64 bit mode.
After we have loaded the Global Descriptor Table, we must enable PAE mode by putting the value of the cr4 register into eax, setting 5 bit in it and loading it again into cr4 :
movl %cr4, %eax
orl $X86_CR4_PAE, %eax
movl %eax, %cr4Now we are almost finished with all preparations before we can move into 64-bit mode. The last step is to build page tables, but before that, here is some information about long mode.
Long mode is the native mode for x86_64 processors. First of all let's look at some differences between x86_64 and x86.
It provides features such as:
- New 8 general purpose registers from
r8tor15+ all general purpose registers are 64-bit now - 64-bit instruction pointer -
RIP - New operating mode - Long mode
- 64-Bit Addresses and Operands
- RIP Relative Addressing (we will see an example if it in the next parts)
Long mode is an extension of legacy protected mode. It consists of two sub-modes:
- 64-bit mode
- compatibility mode
To switch into 64-bit mode we need to do following things:
- enable PAE (we already did it, see above)
- build page tables and load the address of the top level page table into the
cr3register - enable
EFER.LME - enable paging
We already enabled PAE by setting the PAE bit in the cr4 register. Now let's look at paging.
Before we can move into 64-bit mode, we need to build page tables, so, let's look at the building of early 4G boot page tables.
NOTE: I will not describe theory of virtual memory here, if you need to know more about it, see links in the end
The Linux kernel uses 4-level paging, and generally we build 6 page tables:
- One PML4 table
- One PDP table
- Four Page Directory tables
Let's look at the implementation of it. First of all we clear the buffer for the page tables in memory. Every table is 4096 bytes, so we need 24 kilobytes buffer:
leal pgtable(%ebx), %edi
xorl %eax, %eax
movl $((4096*6)/4), %ecx
rep stoslWe put the address stored in ebx (remember that ebx contains the address to relocate the kernel for decompression) with pgtable offset to the edi register. pgtable is defined in the end of head_64.S and looks:
.section ".pgtable","a",@nobits
.balign 4096
pgtable:
.fill 6*4096, 1, 0It is in the .pgtable section and its size is 24 kilobytes. After we put the address in edi, we zero out the eax register and write zeros to the buffer with the rep stosl instruction.
Now we can build the top level page table - PML4 - with:
leal pgtable + 0(%ebx), %edi
leal 0x1007 (%edi), %eax
movl %eax, 0(%edi)Here we get the address stored in the ebx with pgtable offset and put it in edi. Next we put this address with offset 0x1007 in the eax register. 0x1007 is 4096 bytes (size of the PML4) + 7 (PML4 entry flags - PRESENT+RW+USER) and puts eax in edi. After this manipulation edi will contain the address of the first Page Directory Pointer Entry with flags - PRESENT+RW+USER.
In the next step we build 4 Page Directory entries in the Page Directory Pointer table, where the first entry will be with 0x7 flags and the others with 0x8:
leal pgtable + 0x1000(%ebx), %edi
leal 0x1007(%edi), %eax
movl $4, %ecx
1: movl %eax, 0x00(%edi)
addl $0x00001000, %eax
addl $8, %edi
decl %ecx
jnz 1bWe put the base address of the page directory pointer table in edi and the address of the first page directory pointer entry in eax. Put 4 in the ecx register, it will be a counter in the following loop and write the address of the first page directory pointer table entry to the edi register.
After this edi will contain the address of the first page directory pointer entry with flags 0x7. Next we just calculate the address of following page directory pointer entries with flags 0x8 and write their addresses to edi.
The next step is building the 2048 page table entries by 2 megabytes:
leal pgtable + 0x2000(%ebx), %edi
movl $0x00000183, %eax
movl $2048, %ecx
1: movl %eax, 0(%edi)
addl $0x00200000, %eax
addl $8, %edi
decl %ecx
jnz 1bHere we do almost the same as in the previous example, except the first entry will be with flags - $0x00000183 - PRESENT + WRITE + MBZ and all other entries with 0x8. In the end we will have 2048 pages by 2 megabytes.
Our early page table structure are done, it maps 4 gigabytes of memory and now we can put the address of the high-level page table - PML4 - in cr3 control register:
leal pgtable(%ebx), %eax
movl %eax, %cr3That's all. Now we can see transition to the long mode.
First of all we need to set the EFER.LME flag in the MSR to 0xC0000080:
movl $MSR_EFER, %ecx
rdmsr
btsl $_EFER_LME, %eax
wrmsrHere we put the MSR_EFER flag (which is defined in arch/x86/include/uapi/asm/msr-index.h) in the ecx register and call rdmsr instruction which reads the MSR register. After rdmsr executes, we will have the resulting data in edx:eax which depends on the ecx value. We check the EFER_LME bit with the btsl instruction and write data from eax to the MSR register with the wrmsr instruction.
In the next step we push the address of the kernel segment code to the stack (we defined it in the GDT) and put the address of the startup_64 routine in eax.
pushl $__KERNEL_CS
leal startup_64(%ebp), %eaxAfter this we push this address to the stack and enable paging by setting PG and PE bits in the cr0 register:
movl $(X86_CR0_PG | X86_CR0_PE), %eax
movl %eax, %cr0and call:
lretRemember that we pushed the address of the startup_64 function to the stack in the previous step, and after the lret instruction, the CPU extracts the address of it and jumps there.
After all of these steps we're finally in 64-bit mode:
.code64
.org 0x200
ENTRY(startup_64)
....
....
....That's all!
This is the end of the fourth part linux kernel booting process. If you have questions or suggestions, ping me in twitter 0xAX, drop me email or just create an issue.
In the next part we will see kernel decompression and many more.
Please note that English is not my first language and I am really sorry for any inconvenience. If you found any mistakes please send me PR to linux-internals.