[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/core/virtaddr.S

@@ -32,13 +32,13 @@ _virt_to_phys:
 	pushl	%ebp
 
 	/* Change return address to a physical address */
-	movl	virt_offset, %ebp
+	movl	VIRTUAL(virt_offset), %ebp
 	addl	%ebp, 12(%esp)
 
 	/* Switch to physical code segment */
 	cli
 	pushl	$PHYSICAL_CS
-	leal	1f(%ebp), %eax
+	leal	VIRTUAL(1f)(%ebp), %eax
 	pushl	%eax
 	lret
 1:
@@ -78,7 +78,7 @@ _phys_to_virt:
 
 	/* Switch to virtual code segment */
 	cli
-	ljmp	$VIRTUAL_CS, $1f
+	ljmp	$VIRTUAL_CS, $VIRTUAL(1f)
 1:
 	/* Reload data segment registers */
 	movl	$VIRTUAL_DS, %eax
@@ -88,7 +88,7 @@ _phys_to_virt:
 	movl	%eax, %gs
 
 	/* Reload stack segment and adjust %esp */
-	movl	virt_offset, %ebp
+	movl	VIRTUAL(virt_offset), %ebp
 	movl	%eax, %ss
 	subl	%ebp, %esp
 
@@ -134,7 +134,7 @@ _intr_to_virt:
 
 	/* Reload stack segment and adjust %esp if necessary */
 	je	1f
-	movl	virt_offset, %ebp
+	movl	VIRTUAL(virt_offset), %ebp
 	movl	%eax, %ss
 	subl	%ebp, %esp
 1: