x86 architecture documentation

This section contains documentation about coreboot on x86 architecture.

State of x86_64 support

At the moment there’s only experimental x86_64 support. The emulation/qemu-i440fx and emulation/qemu-q35 boards do support ARCH_RAMSTAGE_X86_64 , ARCH_POSTCAR_X86_64 and ARCH_ROMSTAGE_X86_64.

In order to add support for x86_64 the following assumptions were made:

  • The CPU supports long mode
  • All memory returned by malloc must be below 4GiB in physical memory
  • All code that is to be run must be below 4GiB in physical memory
  • The high dword of pointers is always zero
  • The reference implementation is qemu
  • The CPU supports 1GiB hugepages
  • x86 payloads are loaded below 4GiB in physical memory and are jumped to in protected mode

Assumptions for all stages using the reference implementation

  • 0-4GiB are identity mapped using 2MiB-pages as WB
  • Memory above 4GiB isn’t accessible
  • page tables reside in memory mapped ROM
  • A stage can install new page tables in RAM

Page tables

Page tables are generated by a tool in util/pgtblgen/pgtblgen. It writes the page tables to a file which is then included into the CBFS as file called pagetables.

To generate the static page tables it must know the physical address where to place the file.

The page tables contains the following structure:

  • PML4E pointing to PDPE
  • PDPE with $n entries each pointing to PDE
  • $n PDEs with 512 entries each

At the moment $n is 4, which results in identity mapping the lower 4 GiB.

Basic x86_64 support

Basic support for x86_64 has been implemented for QEMU mainboard target.

Reference implementation

The reference implementation is

TODO

  • Identity map memory above 4GiB in ramstage

Future work

  1. Fine grained page tables for SMM:
    • Must not have execute and write permissions for the same page.
    • Must allow only that TSEG pages can be marked executable
    • Must reside in SMRAM
  2. Support 64bit PCI BARs above 4GiB
  3. Place and run code above 4GiB

Porting other boards

  • Fix compilation errors
  • Test how well CAR works with x86_64 and paging
  • Improve mode switches
  • Test libgfxinit / VGA Option ROMs / FSP