mirror of https://github.com/ipxe/ipxe.git
d1562c38a6
The bulk of the iPXE binary (the .textdata section) is physically relocated at runtime to the top of the 32-bit address space in order to allow space for an OS to be loaded. The relocation is achieved with the assistance of segmentation: we adjust the code and data segment bases so that the link-time addresses remain valid. Segmentation is not available (for normal code and data segments) in long mode. We choose to compile the C code with -mcmodel=kernel and use a link-time address of 0xffffffffeb000000. This choice allows us to identity-map the entirety of the 32-bit address space, and to alias our chosen link-time address to the physical location of our .textdata section. (This requires the .textdata section to always be aligned to a page boundary.) We simultaneously choose to set the 32-bit virtual address segment bases such that the link-time addresses may simply be truncated to 32 bits in order to generate a valid 32-bit virtual address. This allows symbols in .textdata to be trivially accessed by both 32-bit and 64-bit code. There is no (sensible) way in 32-bit assembly code to generate the required R_X86_64_32S relocation records for these truncated symbols. However, subtracting the fixed constant 0xffffffff00000000 has the same effect as truncation, and can be represented in a standard R_X86_64_32 relocation record. We define the VIRTUAL() macro to abstract away this truncation operation, and apply it to all references by 32-bit (or 16-bit) assembly code to any symbols within the .textdata section. We define "virt_offset" for a 64-bit build as "the value to be added to an address within .textdata in order to obtain its physical address". With this definition, the low 32 bits of "virt_offset" can be treated by 32-bit code as functionally equivalent to "virt_offset" in a 32-bit build. We define "text16" and "data16" for a 64-bit build as the physical addresses of the .text16 and .data16 sections. Since a physical address within the 32-bit address space may be used directly as a 64-bit virtual address (thanks to the identity map), this definition provides the most natural access to variables in .text16 and .data16. Note that this requires a minor adjustment in prot_to_real(), which accesses .text16 using 32-bit virtual addresses. Signed-off-by: Michael Brown <mcb30@ipxe.org> |
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README
iPXE README File Quick start guide: cd src make For any more detailed instructions, see http://ipxe.org