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x86: document new bzImage fields

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Document the new bzImage fields for kernel memory placement.

[ Impact: adds documentation ]

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
This commit is contained in:
H. Peter Anvin 2009-05-11 16:06:23 -07:00
parent 37ba7ab5e3
commit d297366ba6
1 changed files with 65 additions and 4 deletions
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69
Documentation/x86/boot.txt
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@ -50,6 +50,11 @@ Protocol 2.08: (Kernel 2.6.26) Added crc32 checksum and ELF format
Protocol 2.09: (Kernel 2.6.26) Added a field of 64-bit physical
pointer to single linked list of struct setup_data.
Protocol 2.10: (Kernel 2.6.31) A protocol for relaxed alignment
beyond the kernel_alignment added, new init_size and
pref_address fields.
**** MEMORY LAYOUT
The traditional memory map for the kernel loader, used for Image or
@ -173,7 +178,7 @@ Offset Proto Name Meaning
022C/4 2.03+ ramdisk_max Highest legal initrd address
0230/4 2.05+ kernel_alignment Physical addr alignment required for kernel
0234/1 2.05+ relocatable_kernel Whether kernel is relocatable or not
0235/1 N/A pad2 Unused
0235/1 2.10+ min_alignment Minimum alignment, as a power of two
0236/2 N/A pad3 Unused
0238/4 2.06+ cmdline_size Maximum size of the kernel command line
023C/4 2.07+ hardware_subarch Hardware subarchitecture
@ -182,6 +187,8 @@ Offset Proto Name Meaning
024C/4 2.08+ payload_length Length of kernel payload
0250/8 2.09+ setup_data 64-bit physical pointer to linked list
of struct setup_data
0258/8 2.10+ pref_address Preferred loading address
0260/4 2.10+ init_size Linear memory required during initialization
(1) For backwards compatibility, if the setup_sects field contains 0, the
real value is 4.
@ -482,11 +489,19 @@ Protocol: 2.03+
0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
Field name: kernel_alignment
Type: read (reloc)
Type: read/modify (reloc)
Offset/size: 0x230/4
Protocol: 2.05+
Protocol: 2.05+ (read), 2.10+ (modify)
Alignment unit required by the kernel (if relocatable_kernel is true.)
Alignment unit required by the kernel (if relocatable_kernel is
true.) A relocatable kernel that is loaded at an alignment
incompatible with the value in this field will be realigned during
kernel initialization.
Starting with protocol version 2.10, this reflects the kernel
alignment preferred for optimal performance; it is possible for the
loader to modify this field to permit a lesser alignment. See the
min_alignment and pref_address field below.
Field name: relocatable_kernel
Type: read (reloc)
@ -498,6 +513,22 @@ Protocol: 2.05+
After loading, the boot loader must set the code32_start field to
point to the loaded code, or to a boot loader hook.
Field name: min_alignment
Type: read (reloc)
Offset/size: 0x235/1
Protocol: 2.10+
This field, if nonzero, indicates as a power of two the minimum
alignment required, as opposed to preferred, by the kernel to boot.
If a boot loader makes use of this field, it should update the
kernel_alignment field with the alignment unit desired; typically:
kernel_alignment = 1 << min_alignment
There may be a considerable performance cost with an excessively
misaligned kernel. Therefore, a loader should typically try each
power-of-two alignment from kernel_alignment down to this alignment.
Field name: cmdline_size
Type: read
Offset/size: 0x238/4
@ -582,6 +613,36 @@ Protocol: 2.09+
sure to consider the case where the linked list already contains
entries.
Field name: pref_address
Type: read (reloc)
Offset/size: 0x258/8
Protocol: 2.10+
This field, if nonzero, represents a preferred load address for the
kernel. A relocating bootloader should attempt to load at this
address if possible.
A non-relocatable kernel will unconditionally move itself and to run
at this address.
Field name: init_size
Type: read
Offset/size: 0x25c/4
This field indicates the amount of linear contiguous memory starting
at the kernel runtime start address that the kernel needs before it
is capable of examining its memory map. This is not the same thing
as the total amount of memory the kernel needs to boot, but it can
be used by a relocating boot loader to help select a safe load
address for the kernel.
The kernel runtime start address is determined by the following algorithm:
if (relocatable_kernel)
runtime_start = align_up(load_address, kernel_alignment)
else
runtime_start = pref_address
**** THE IMAGE CHECKSUM