x86_64 imporatant information

[Ruslan Nikolaev]

In my previous post I forgot to say about the most imporatant reason to use negative addresses.

 

Take a look on the following information:

 

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/usr/src/linux/vmlinux:     file format elf64-x86-64

Sections:
Idx Name          Size      VMA               LMA               File off  Algn
  0 .text         00368c04  ffffffff80100000  ffffffff80100000  00100000  2**6
                  CONTENTS, ALLOC, LOAD, READONLY, CODE
  1 __ex_table    00005470  ffffffff80468c10  ffffffff80468c10  00468c10  2**3
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
  2 .rodata       000582ea  ffffffff8046e080  ffffffff8046e080  0046e080  2**5
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
  3 .pci_fixup    000007b0  ffffffff804c6370  ffffffff804c6370  004c6370  2**4
                  CONTENTS, ALLOC, LOAD, DATA

...

----------------------

 

It seems that VMA and LMA addresses are the same for Linux ELF under x86_64. I never interested such information for x86 and don't know how it will be for x86.

 

I think it's just because we need to use special memory model "-mcmodel=kernel" that's designed especially for kernel. Does we have problems when LMA != VMA and using this memory model? I can't find such information...

 

From x86_64 ABI:

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The AMD64 architecture usually does not allow to encode arbitrary 64-bit constants

as immediate operand of the instruction. Most instructions accept 32-bit

immediates that are sign extended to the 64-bit ones. Additionally the 32-bit operations

with register destinations implicitly perform zero extension making loads

of 64-bit immediates with upper half set to 0 even cheaper.

Additionally the branch instructions accept 32-bit immediate operands that

are sign extended and used to adjust instruction pointer. Similarly an instruction

pointer relative addressing mode exists for data accesses with equivalent limitations.

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