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>

4 files changed, 98 insertions, 1 deletions

diff --git a/arch/arm/kernel/armksyms.c b/arch/arm/kernel/armksyms.c
index e5e1e5387678..9615423c37dd 100644
--- a/arch/arm/kernel/armksyms.c
+++ b/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
diff --git a/arch/arm/kernel/head.S b/arch/arm/kernel/head.S
index 03a588b6e15c..1db8ead2e331 100644
--- a/arch/arm/kernel/head.S
+++ b/arch/arm/kernel/head.S
@@ -98,6 +98,9 @@ ENTRY(stext)
 #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"
diff --git a/arch/arm/kernel/module.c b/arch/arm/kernel/module.c
index 2cfe8161b478..c5679f6d9f64 100644
--- a/arch/arm/kernel/module.c
+++ b/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;
 
@@ -315,6 +331,11 @@ int module_finalize(const Elf32_Ehdr *hdr, const Elf_Shdr *sechdrs,
 					         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;
 }
 
diff --git a/arch/arm/kernel/vmlinux.lds.S b/arch/arm/kernel/vmlinux.lds.S
index 86b66f3f2031..45b5651777ee 100644
--- a/arch/arm/kernel/vmlinux.lds.S
+++ b/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