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author | Michael Brown | 2016-02-19 02:50:13 +0100 |
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committer | Michael Brown | 2016-02-19 03:58:09 +0100 |
commit | d1562c38a6882a129998935cd63d09ab18f77add (patch) | |
tree | e834487ca48085ca2f62d200419e6a839301a514 /src/arch/x86_64/Makefile.efi | |
parent | [relocate] Preserve page alignment during relocation (diff) | |
download | ipxe-d1562c38a6882a129998935cd63d09ab18f77add.tar.gz ipxe-d1562c38a6882a129998935cd63d09ab18f77add.tar.xz ipxe-d1562c38a6882a129998935cd63d09ab18f77add.zip |
[librm] Prepare for long-mode memory map
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>
Diffstat (limited to 'src/arch/x86_64/Makefile.efi')
-rw-r--r-- | src/arch/x86_64/Makefile.efi | 4 |
1 files changed, 4 insertions, 0 deletions
diff --git a/src/arch/x86_64/Makefile.efi b/src/arch/x86_64/Makefile.efi index 12408f86..0041bb8f 100644 --- a/src/arch/x86_64/Makefile.efi +++ b/src/arch/x86_64/Makefile.efi @@ -1,5 +1,9 @@ # -*- makefile -*- : Force emacs to use Makefile mode +# Use %rip-relative addressing wherever possible. +# +CFLAGS += -fpie + # EFI probably doesn't guarantee us a red zone, so let's not rely on it. # CFLAGS += -mno-red-zone |