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fix boot-4

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Alexander Kuleshov 2018-04-04 00:20:22 +06:00
parent 4382670c38
commit e735b508d1
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1 changed files with 12 additions and 8 deletions
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16
Booting/linux-bootstrap-4.md
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@ -151,7 +151,7 @@ Bit 6 (write): KEEP_SEGMENTS
So, if the `KEEP_SEGMENTS` bit is not set in the `loadflags`, we need to set `ds`, `ss` and `es` segment registers to the index of data segment with base `0`. That we do:
```C
testb $(1 << 6), BP_loadflags(%esi)
testb $KEEP_SEGMENTS, BP_loadflags(%esi)
jnz 1f
cli
@ -337,8 +337,6 @@ When we are using position-independent code an address is obtained by adding the
jge 1f
#endif
movl $LOAD_PHYSICAL_ADDR, %ebx
1:
addl $z_extract_offset, %ebx
```
Remember that the value of the `ebp` register is the physical address of the `startup_32` label. If the `CONFIG_RELOCATABLE` kernel configuration option is enabled during kernel configuration, we put this address in the `ebx` register, align it to a multiple of `2MB` and compare it with the `LOAD_PHYSICAL_ADDR` value. The `LOAD_PHYSICAL_ADDR` macro is defined in the [arch/x86/include/asm/boot.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/include/asm/boot.h) header file and it looks like this:
@ -354,9 +352,10 @@ As we can see it just expands to the aligned `CONFIG_PHYSICAL_ALIGN` value which
After all of these calculations, we will have `ebp` which contains the address where we loaded it and `ebx` set to the address of where kernel will be moved after decompression. But that is not the end. The compressed kernel image should be moved to the end of the decompression buffer to simplify calculations where kernel will be located later. For this:
```assembly
movl BP_init_size(%esi), %eax
subl $_end, %eax
addl %eax, %ebx
1:
movl BP_init_size(%esi), %eax
subl $_end, %eax
addl %eax, %ebx
```
we put value from the `boot_params.BP_init_size` (or kernel setup header value from the `hdr.init_size`) to the `eax` register. The `BP_init_size` contains larger value between compressed and uncompressed [vmlinux](https://en.wikipedia.org/wiki/Vmlinux). Next we subtract address of the `_end` symbol from this value and add the result of subtraction to `ebx` register which will stores base address for kernel decompression.
@ -377,6 +376,11 @@ To understand the magic with `gdt` offsets we need to look at the definition of
```assembly
.data
gdt64:
.word gdt_end - gdt
.long 0
.word 0
.quad 0
gdt:
.word gdt_end - gdt
.long gdt