[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>

6 files changed, 96 insertions, 35 deletions

diff --git a/src/arch/x86/core/virtaddr.S b/src/arch/x86/core/virtaddr.S
index 42559157..45beb164 100644
--- a/src/arch/x86/core/virtaddr.S
+++ b/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:
diff --git a/src/arch/x86/include/librm.h b/src/arch/x86/include/librm.h
index 97461640..fc31c503 100644
--- a/src/arch/x86/include/librm.h
+++ b/src/arch/x86/include/librm.h
@@ -7,7 +7,6 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
  *
  * Don't change these unless you really know what you're doing.
  */
-
 #define VIRTUAL_CS 0x08
 #define VIRTUAL_DS 0x10
 #define PHYSICAL_CS 0x18
@@ -16,6 +15,40 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #define REAL_DS 0x30
 #define P2R_DS 0x38
 
+/* Calculate symbol address within VIRTUAL_CS or VIRTUAL_DS
+ *
+ * In a 64-bit build, we set the bases of VIRTUAL_CS and VIRTUAL_DS
+ * such that truncating a .textdata symbol value to 32 bits gives a
+ * valid 32-bit virtual address.
+ *
+ * The C code is compiled with -mcmodel=kernel and so we must place
+ * all .textdata symbols within the negative 2GB of the 64-bit address
+ * space.  Consequently, all .textdata symbols will have the MSB set
+ * after truncation to 32 bits.  This means that a straightforward
+ * R_X86_64_32 relocation record for the symbol will fail, since the
+ * truncated symbol value will not correctly zero-extend to the
+ * original 64-bit value.
+ *
+ * Using an R_X86_64_32S relocation record would work, but there is no
+ * (sensible) way to generate these relocation records within 32-bit
+ * or 16-bit code.
+ *
+ * The simplest solution is to generate an R_X86_64_32 relocation
+ * record with an addend of (-0xffffffff00000000).  Since all
+ * .textdata symbols are within the negative 2GB of the 64-bit address
+ * space, this addend acts to effectively truncate the symbol to 32
+ * bits, thereby matching the semantics of the R_X86_64_32 relocation
+ * records generated for 32-bit and 16-bit code.
+ *
+ * In a 32-bit build, this problem does not exist, and we can just use
+ * the .textdata symbol values directly.
+ */
+#ifdef __x86_64__
+#define VIRTUAL(address) ( (address) - 0xffffffff00000000 )
+#else
+#define VIRTUAL(address) (address)
+#endif
+
 #ifdef ASSEMBLY
 
 /**
@@ -24,7 +57,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
  * @v function		C function
  */
 .macro virtcall function
-	pushl	$\function
+	pushl	$VIRTUAL(\function)
 	call	prot_call
 .endm
 
@@ -42,7 +75,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
  * @v function		C function
  */
 #define VIRT_CALL( function )						\
-	"pushl $( " #function " )\n\t"					\
+	"pushl $( " _S2 ( VIRTUAL ( function ) ) " )\n\t"		\
 	"call prot_call\n\t"
 
 /* Variables in librm.S */
diff --git a/src/arch/x86/transitions/librm.S b/src/arch/x86/transitions/librm.S
index 42cfb991..495f272d 100644
--- a/src/arch/x86/transitions/librm.S
+++ b/src/arch/x86/transitions/librm.S
@@ -19,8 +19,22 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL )
 #define SIZEOF_REAL_MODE_REGS	( SIZEOF_I386_SEG_REGS + SIZEOF_I386_REGS )
 #define SIZEOF_I386_FLAGS	4
 #define SIZEOF_I386_ALL_REGS	( SIZEOF_REAL_MODE_REGS + SIZEOF_I386_FLAGS )
-	
-	.arch i386
+
+/* Size of an address */
+#ifdef __x86_64__
+#define SIZEOF_ADDR 8
+#else
+#define SIZEOF_ADDR 4
+#endif
+
+/* Selectively assemble code for 32-bit/64-bit builds */
+#ifdef __x86_64__
+#define if32 if 0
+#define if64 if 1
+#else
+#define if32 if 1
+#define if64 if 0
+#endif
 
 /****************************************************************************
  * Global descriptor table
@@ -126,7 +140,7 @@ rm_sp:	.word 0
 rm_ss:	.word 0
 
 	.section ".data.pm_esp", "aw", @progbits
-pm_esp:	.long _estack
+pm_esp:	.long VIRTUAL(_estack)
 
 /****************************************************************************
  * Virtual address offsets
@@ -137,9 +151,9 @@ pm_esp:	.long _estack
  ****************************************************************************
  */
 	.struct 0
-VA_VIRT_OFFSET:	.space	4
-VA_TEXT16:	.space	4
-VA_DATA16:	.space	4
+VA_VIRT_OFFSET:	.space	SIZEOF_ADDR
+VA_TEXT16:	.space	SIZEOF_ADDR
+VA_DATA16:	.space	SIZEOF_ADDR
 VA_SIZE:
 	.previous
 
@@ -168,7 +182,7 @@ virt_addrs:	.space	VA_SIZE
  * Parameters:
  *   %cs : .text16 segment
  *   %ds : .data16 segment
- *   %edi : Physical base of protected-mode code (virt_offset)
+ *   %edi : Physical base of protected-mode code
  ****************************************************************************
  */
 	.section ".text16.init_librm", "ax", @progbits
@@ -181,7 +195,9 @@ init_librm:
 	pushl	%edi
 
 	/* Store rm_virt_offset and set up virtual_cs and virtual_ds segments */
+	subl	$VIRTUAL(_textdata), %edi
 	movl	%edi, rm_virt_offset
+.if64 ;	setae	(rm_virt_offset+4) ; .endif
 	movl	%edi, %eax
 	movw	$virtual_cs, %bx
 	call	set_seg_base
@@ -195,7 +211,7 @@ init_librm:
 	shll	$4, %eax
 	movw	$real_cs, %bx
 	call	set_seg_base
-	subl	%edi, %eax
+.if32 ;	subl	%edi, %eax ; .endif
 	movl	%eax, rm_text16
 
 	/* Store rm_ds and rm_data16, set up real_ds segment and GDT base */
@@ -207,7 +223,7 @@ init_librm:
 	call	set_seg_base
 	movl	%eax, gdt_base
 	addl	$gdt, gdt_base
-	subl	%edi, %eax
+.if32 ;	subl	%edi, %eax ; .endif
 	movl	%eax, rm_data16
 
 	/* Switch to protected mode */
@@ -221,7 +237,7 @@ init_librm_pmode:
 	movw	$REAL_DS, %ax
 	movw	%ax, %ds
 	movl	$rm_virt_addrs, %esi
-	movl	$virt_addrs, %edi
+	movl	$VIRTUAL(virt_addrs), %edi
 	movl	$( VA_SIZE / 4 ), %ecx
 	rep movsl
 	popw	%ds
@@ -312,7 +328,7 @@ real_to_prot:
 	movl	%cr0, %eax
 	orb	$CR0_PE, %al
 	movl	%eax, %cr0
-	data32 ljmp	$VIRTUAL_CS, $r2p_pmode
+	data32 ljmp	$VIRTUAL_CS, $VIRTUAL(r2p_pmode)
 	.section ".text.real_to_prot", "ax", @progbits
 	.code32
 r2p_pmode:
@@ -323,15 +339,15 @@ r2p_pmode:
 	movw	%ax, %fs
 	movw	%ax, %gs
 	movw	%ax, %ss
-	movl	pm_esp, %esp
+	movl	VIRTUAL(pm_esp), %esp
 
 	/* Load protected-mode interrupt descriptor table */
-	lidt	idtr
+	lidt	VIRTUAL(idtr)
 
 	/* Record real-mode %ss:sp (after removal of data) */
-	movw	%bp, rm_ss
+	movw	%bp, VIRTUAL(rm_ss)
 	addl	%ecx, %edx
-	movw	%dx, rm_sp
+	movw	%dx, VIRTUAL(rm_sp)
 
 	/* Move data from RM stack to PM stack */
 	subl	%ecx, %esp
@@ -365,7 +381,8 @@ r2p_pmode:
 	.code32
 prot_to_real:
 	/* Copy real-mode global descriptor table register to RM code segment */
-	movl	text16, %edi
+	movl	VIRTUAL(text16), %edi
+.if64 ;	subl	VIRTUAL(virt_offset), %edi ; .endif
 	leal	rm_gdtr(%edi), %edi
 	movsw
 	movsl
@@ -377,20 +394,20 @@ prot_to_real:
 	addl	$4, %ecx
 
 	/* Real-mode %ss:sp => %ebp:edx and virtual address => %edi */
-	movzwl	rm_ss, %ebp
-	movzwl	rm_sp, %edx
+	movzwl	VIRTUAL(rm_ss), %ebp
+	movzwl	VIRTUAL(rm_sp), %edx
 	subl	%ecx, %edx
 	movl	%ebp, %eax
 	shll	$4, %eax
 	leal	(%eax,%edx), %edi
-	subl	virt_offset, %edi
+	subl	VIRTUAL(virt_offset), %edi
 
 	/* Move data from PM stack to RM stack */
 	movl	%esp, %esi
 	rep movsb
 
 	/* Record protected-mode %esp (after removal of data) */
-	movl	%esi, pm_esp
+	movl	%esi, VIRTUAL(pm_esp)
 
 	/* Load real-mode segment limits */
 	movw	$P2R_DS, %ax
@@ -512,7 +529,7 @@ prot_call:
 
 	/* Switch to protected mode and move register dump to PM stack */
 	movl	$PC_OFFSET_END, %ecx
-	pushl	$pc_pmode
+	pushl	$VIRTUAL(pc_pmode)
 	jmp	real_to_prot
 	.section ".text.prot_call", "ax", @progbits
 	.code32
@@ -589,7 +606,7 @@ real_call:
 	/* Switch to real mode and move register dump to RM stack  */
 	movl	$( RC_OFFSET_REGS_END + 4 /* function pointer copy */ ), %ecx
 	pushl	$rc_rmode
-	movl	$rm_default_gdtr_idtr, %esi
+	movl	$VIRTUAL(rm_default_gdtr_idtr), %esi
 	jmp	prot_to_real
 	.section ".text16.real_call", "ax", @progbits
 	.code16
@@ -605,7 +622,7 @@ rc_rmode:
 
 	/* Switch to protected mode and move register dump back to PM stack */
 	movl	$RC_OFFSET_REGS_END, %ecx
-	pushl	$rc_pmode
+	pushl	$VIRTUAL(rc_pmode)
 	jmp	real_to_prot
 	.section ".text.real_call", "ax", @progbits
 	.code32
@@ -665,6 +682,8 @@ flatten_dummy:
  * May be entered with either physical or virtual stack segment.
  ****************************************************************************
  */
+	.section ".text.interrupt_wrapper", "ax", @progbits
+	.code32
 	.globl interrupt_wrapper
 interrupt_wrapper:
 	/* Preserve segment registers and original %esp */
diff --git a/src/arch/x86_64/Makefile b/src/arch/x86_64/Makefile
index 48c0aa1a..246905cd 100644
--- a/src/arch/x86_64/Makefile
+++ b/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
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
diff --git a/src/arch/x86_64/Makefile.pcbios b/src/arch/x86_64/Makefile.pcbios
index dfb8db0a..ba4c8d8d 100644
--- a/src/arch/x86_64/Makefile.pcbios
+++ b/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