ARM: P2V: introduce phys_to_virt/virt_to_phys runtime patching

This idea came from Nicolas, Eric Miao produced an initial version,
which was then rewritten into this.

Patch the physical to virtual translations at runtime.  As we modify
the code, this makes it incompatible with XIP kernels, but allows us
to achieve this with minimal loss of performance.

As many translations are of the form:

	physical = virtual + (PHYS_OFFSET - PAGE_OFFSET)
	virtual = physical - (PHYS_OFFSET - PAGE_OFFSET)

we generate an 'add' instruction for __virt_to_phys(), and a 'sub'
instruction for __phys_to_virt().  We calculate at run time (PHYS_OFFSET
- PAGE_OFFSET) by comparing the address prior to MMU initialization with
where it should be once the MMU has been initialized, and place this
constant into the above add/sub instructions.

Once we have (PHYS_OFFSET - PAGE_OFFSET), we can calculate the real
PHYS_OFFSET as PAGE_OFFSET is a build-time constant, and save this for
the C-mode PHYS_OFFSET variable definition to use.

At present, we are unable to support Realview with Sparsemem enabled
as this uses a complex mapping function, and MSM as this requires a
constant which will not fit in our math instruction.

Add a module version magic string for this feature to prevent
incompatible modules being loaded.

Tested-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Tested-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/Kconfig

@@ -191,6 +191,19 @@ config VECTORS_BASE
 	help
 	  The base address of exception vectors.
 
+config ARM_PATCH_PHYS_VIRT
+	bool "Patch physical to virtual translations at runtime (EXPERIMENTAL)"
+	depends on EXPERIMENTAL
+	depends on !XIP_KERNEL && !THUMB2_KERNEL && MMU
+	depends on !ARCH_MSM
+	depends on !ARCH_REALVIEW || !SPARSEMEM
+	help
+	  Patch phys-to-virt translation functions at runtime according to
+	  the position of the kernel in system memory.
+
+	  This can only be used with non-XIP, non-Thumb2, MMU kernels where
+	  the base of physical memory is at a 16MB boundary.
+
 source "init/Kconfig"
 
 source "kernel/Kconfig.freezer"

arch/arm/include/asm/memory.h

@@ -24,8 +24,6 @@
  */
 #define UL(x) _AC(x, UL)
 
-#define PHYS_OFFSET	PLAT_PHYS_OFFSET
-
 #ifdef CONFIG_MMU
 
 /*
@@ -134,16 +132,6 @@
 #define DTCM_OFFSET	UL(0xfffe8000)
 #endif
 
-/*
- * Physical vs virtual RAM address space conversion.  These are
- * private definitions which should NOT be used outside memory.h
- * files.  Use virt_to_phys/phys_to_virt/__pa/__va instead.
- */
-#ifndef __virt_to_phys
-#define __virt_to_phys(x)	((x) - PAGE_OFFSET + PHYS_OFFSET)
-#define __phys_to_virt(x)	((x) - PHYS_OFFSET + PAGE_OFFSET)
-#endif
-
 /*
  * Convert a physical address to a Page Frame Number and back
  */
@@ -158,6 +146,49 @@
 
 #ifndef __ASSEMBLY__
 
+/*
+ * Physical vs virtual RAM address space conversion.  These are
+ * private definitions which should NOT be used outside memory.h
+ * files.  Use virt_to_phys/phys_to_virt/__pa/__va instead.
+ */
+#ifndef __virt_to_phys
+#ifdef CONFIG_ARM_PATCH_PHYS_VIRT
+
+extern unsigned long __pv_phys_offset;
+#define PHYS_OFFSET __pv_phys_offset
+
+#define __pv_stub(from,to,instr)			\
+	__asm__("@ __pv_stub\n"				\
+	"1:	" instr "	%0, %1, %2\n"		\
+	"	.pushsection .pv_table,\"a\"\n"		\
+	"	.long	1b\n"				\
+	"	.popsection\n"				\
+	: "=r" (to)					\
+	: "r" (from), "I" (0x81000000))
+
+static inline unsigned long __virt_to_phys(unsigned long x)
+{
+	unsigned long t;
+	__pv_stub(x, t, "add");
+	return t;
+}
+
+static inline unsigned long __phys_to_virt(unsigned long x)
+{
+	unsigned long t;
+	__pv_stub(x, t, "sub");
+	return t;
+}
+#else
+#define __virt_to_phys(x)	((x) - PAGE_OFFSET + PHYS_OFFSET)
+#define __phys_to_virt(x)	((x) - PHYS_OFFSET + PAGE_OFFSET)
+#endif
+#endif
+
+#ifndef PHYS_OFFSET
+#define PHYS_OFFSET	PLAT_PHYS_OFFSET
+#endif
+
 /*
  * The DMA mask corresponding to the maximum bus address allocatable
  * using GFP_DMA.  The default here places no restriction on DMA

arch/arm/include/asm/module.h

@@ -25,8 +25,19 @@ struct mod_arch_specific {
 };
 
 /*
- * Include the ARM architecture version.
+ * Add the ARM architecture version to the version magic string
  */
-#define MODULE_ARCH_VERMAGIC	"ARMv" __stringify(__LINUX_ARM_ARCH__) " "
+#define MODULE_ARCH_VERMAGIC_ARMVSN "ARMv" __stringify(__LINUX_ARM_ARCH__) " "
+
+/* Add __virt_to_phys patching state as well */
+#ifdef CONFIG_ARM_PATCH_PHYS_VIRT
+#define MODULE_ARCH_VERMAGIC_P2V "p2v8 "
+#else
+#define MODULE_ARCH_VERMAGIC_P2V ""
+#endif
+
+#define MODULE_ARCH_VERMAGIC \
+	MODULE_ARCH_VERMAGIC_ARMVSN \
+	MODULE_ARCH_VERMAGIC_P2V
 
 #endif /* _ASM_ARM_MODULE_H */

arch/arm/kernel/armksyms.c

@@ -170,3 +170,7 @@ EXPORT_SYMBOL(mcount);
 #endif
 EXPORT_SYMBOL(__gnu_mcount_nc);
 #endif
+
+#ifdef CONFIG_ARM_PATCH_PHYS_VIRT
+EXPORT_SYMBOL(__pv_phys_offset);
+#endif

arch/arm/kernel/head.S

@@ -97,6 +97,9 @@ ENTRY(stext)
 	bl	__vet_atags
 #ifdef CONFIG_SMP_ON_UP
 	bl	__fixup_smp
+#endif
+#ifdef CONFIG_ARM_PATCH_PHYS_VIRT
+	bl	__fixup_pv_table
 #endif
 	bl	__create_page_tables
 
@@ -438,4 +441,69 @@ smp_on_up:
 
 #endif
 
+#ifdef CONFIG_ARM_PATCH_PHYS_VIRT
+
+/* __fixup_pv_table - patch the stub instructions with the delta between
+ * PHYS_OFFSET and PAGE_OFFSET, which is assumed to be 16MiB aligned and
+ * can be expressed by an immediate shifter operand. The stub instruction
+ * has a form of '(add|sub) rd, rn, #imm'.
+ */
+	__HEAD
+__fixup_pv_table:
+	adr	r0, 1f
+	ldmia	r0, {r3-r5, r7}
+	sub	r3, r0, r3	@ PHYS_OFFSET - PAGE_OFFSET
+	add	r4, r4, r3	@ adjust table start address
+	add	r5, r5, r3	@ adjust table end address
+	str	r8, [r7, r3]!	@ save computed PHYS_OFFSET to __pv_phys_offset
+	mov	r6, r3, lsr #24	@ constant for add/sub instructions
+	teq	r3, r6, lsl #24 @ must be 16MiB aligned
+	bne	__error
+	str	r6, [r7, #4]	@ save to __pv_offset
+	b	__fixup_a_pv_table
+ENDPROC(__fixup_pv_table)
+
+	.align
+1:	.long	.
+	.long	__pv_table_begin
+	.long	__pv_table_end
+2:	.long	__pv_phys_offset
+
+	.text
+__fixup_a_pv_table:
+	b	3f
+2:	ldr	ip, [r7, r3]
+	bic	ip, ip, #0x000000ff
+	orr	ip, ip, r6
+	str	ip, [r7, r3]
+3:	cmp	r4, r5
+	ldrcc	r7, [r4], #4	@ use branch for delay slot
+	bcc	2b
+	mov	pc, lr
+ENDPROC(__fixup_a_pv_table)
+
+ENTRY(fixup_pv_table)
+	stmfd	sp!, {r4 - r7, lr}
+	ldr	r2, 2f			@ get address of __pv_phys_offset
+	mov	r3, #0			@ no offset
+	mov	r4, r0			@ r0 = table start
+	add	r5, r0, r1		@ r1 = table size
+	ldr	r6, [r2, #4]		@ get __pv_offset
+	bl	__fixup_a_pv_table
+	ldmfd	sp!, {r4 - r7, pc}
+ENDPROC(fixup_pv_table)
+
+	.align
+2:	.long	__pv_phys_offset
+
+	.data
+	.globl	__pv_phys_offset
+	.type	__pv_phys_offset, %object
+__pv_phys_offset:
+	.long	0
+	.size	__pv_phys_offset, . - __pv_phys_offset
+__pv_offset:
+	.long	0
+#endif
+
 #include "head-common.S"

arch/arm/kernel/module.c

@@ -268,12 +268,28 @@ struct mod_unwind_map {
 	const Elf_Shdr *txt_sec;
 };
 
+static const Elf_Shdr *find_mod_section(const Elf32_Ehdr *hdr,
+	const Elf_Shdr *sechdrs, const char *name)
+{
+	const Elf_Shdr *s, *se;
+	const char *secstrs = (void *)hdr + sechdrs[hdr->e_shstrndx].sh_offset;
+
+	for (s = sechdrs, se = sechdrs + hdr->e_shnum; s < se; s++)
+		if (strcmp(name, secstrs + s->sh_name) == 0)
+			return s;
+
+	return NULL;
+}
+
+extern void fixup_pv_table(const void *, unsigned long);
+
 int module_finalize(const Elf32_Ehdr *hdr, const Elf_Shdr *sechdrs,
 		    struct module *mod)
 {
+	const Elf_Shdr *s = NULL;
 #ifdef CONFIG_ARM_UNWIND
 	const char *secstrs = (void *)hdr + sechdrs[hdr->e_shstrndx].sh_offset;
-	const Elf_Shdr *s, *sechdrs_end = sechdrs + hdr->e_shnum;
+	const Elf_Shdr *sechdrs_end = sechdrs + hdr->e_shnum;
 	struct mod_unwind_map maps[ARM_SEC_MAX];
 	int i;
 
@@ -314,6 +330,11 @@ int module_finalize(const Elf32_Ehdr *hdr, const Elf_Shdr *sechdrs,
 					         maps[i].unw_sec->sh_size,
 					         maps[i].txt_sec->sh_addr,
 					         maps[i].txt_sec->sh_size);
+#endif
+#ifdef CONFIG_ARM_PATCH_PHYS_VIRT
+	s = find_mod_section(hdr, sechdrs, ".pv_table");
+	if (s)
+		fixup_pv_table((void *)s->sh_addr, s->sh_size);
 #endif
 	return 0;
 }

arch/arm/kernel/vmlinux.lds.S

@@ -57,6 +57,10 @@ SECTIONS
 		__smpalt_end = .;
 #endif
 
+		__pv_table_begin = .;
+			*(.pv_table)
+		__pv_table_end = .;
+
 		INIT_SETUP(16)
 
 		INIT_CALLS