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>

7 files changed, 167 insertions, 15 deletions

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 5cff165b7eb0..4147f76e7988 100644
--- a/arch/arm/Kconfig
+++ b/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"
diff --git a/arch/arm/include/asm/memory.h b/arch/arm/include/asm/memory.h
index 2efec578a62e..7197879e1cb7 100644
--- a/arch/arm/include/asm/memory.h
+++ b/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
 
 /*
@@ -135,16 +133,6 @@
 #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
  */
 #define __phys_to_pfn(paddr)    ((paddr) >> PAGE_SHIFT)
@@ -159,6 +147,49 @@
 #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
  * allocations.  This must be the smallest DMA mask in the system,
diff --git a/arch/arm/include/asm/module.h b/arch/arm/include/asm/module.h
index 12c8e680cbff..d072c21332ee 100644
--- a/arch/arm/include/asm/module.h
+++ b/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 */
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