[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>

src/arch/x86_64/Makefile

@@ -7,10 +7,6 @@ CFLAGS		+= -fstrength-reduce -fomit-frame-pointer
 #
 CFLAGS		+= -falign-jumps=1 -falign-loops=1 -falign-functions=1
 
-# Use %rip-relative addressing wherever possible.
-#
-CFLAGS		+= -fpie
-
 # Force 64-bit code
 #
 CFLAGS		+= -m64

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

src/arch/x86_64/Makefile.pcbios

@@ -1,5 +1,14 @@
 # -*- makefile -*- : Force emacs to use Makefile mode
 
+# Place .textdata in negative 2GB of address space
+#
+CFLAGS		+= -mcmodel=kernel
+LDFLAGS		+= --section-start=.textdata=0xffffffffeb000000
+
+# Assembly code does not respect a red zone.
+#
+CFLAGS		+= -mno-red-zone
+
 # Include generic BIOS Makefile
 #
 MAKEDEPS	+= arch/x86/Makefile.pcbios