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

Known bugs on real hardware

According to Intel x86_64 mode hasn’t been validated in CAR environments. Until now it could be verified on various Intel platforms and no issues have been found.

Known bugs on KVM enabled qemu

The x86_64 reference code runs fine in qemu soft-cpu, but has serious issues when using KVM mode on some machines. The workaround is to not place page-tables in ROM, as done in CB:49228.

Here’s a list of known issues:

  • After entering long mode, the FPU doesn’t work anymore, including accessing MMX registers. It works fine before entering long mode. It works fine when switching back to protected mode. Other registers, like SSE registers, are working fine.
  • Reading from virtual memory, when the page tables are stored in ROM, causes the MMU to abort the “page table walking” mechanism when the lower address bits of the virtual address to be translated have a specific pattern. Instead of loading the correct physical page, the one containing the page tables in ROM will be loaded and used, which breaks code and data as the page table doesn’t contain the expected data. This in turn leads to undefined behaviour whenever the ‘wrong’ address is being read.
  • Disabling paging in compability mode crashes the CPU.
  • Returning from long mode to compability mode crashes the CPU.
  • Entering long mode crashes on AMD host platforms.