tile: define a macro ktext_writable_addr to get writable kernel text address

It is used by kgdb, ftrace, kprobe and jump label, so we factor
this out into a helper routine.

Reviewed-by: Chris Metcalf <[e-mail]>
Signed-off-by: Zhigang Lu <[e-mail]>
Signed-off-by: Chris Metcalf <[e-mail]>

arch/tile/include/asm/page.h

···
 #define virt_to_page(kaddr) pfn_to_page(kaddr_to_pfn((void *)(kaddr)))
 #define page_to_virt(page) pfn_to_kaddr(page_to_pfn(page))
 
+/*
+ * The kernel text is mapped at MEM_SV_START as read-only.  To allow
+ * modifying kernel text, it is also mapped at PAGE_OFFSET as read-write.
+ * This macro converts a kernel address to its writable kernel text mapping,
+ * which is used to modify the text code on a running kernel by kgdb,
+ * ftrace, kprobe, jump label, etc.
+ */
+#define ktext_writable_addr(kaddr) \
+	((unsigned long)(kaddr) - MEM_SV_START + PAGE_OFFSET)
+
 struct mm_struct;
 extern pte_t *virt_to_pte(struct mm_struct *mm, unsigned long addr);
 extern pte_t *virt_to_kpte(unsigned long kaddr);

arch/tile/kernel/ftrace.c

···
 		return -EINVAL;
 
 	/* Operate on writable kernel text mapping. */
-	pc_wr = pc - MEM_SV_START + PAGE_OFFSET;
+	pc_wr = ktext_writable_addr(pc);
 
 	if (probe_kernel_write((void *)pc_wr, &new, MCOUNT_INSN_SIZE))
 		return -EPERM;

arch/tile/kernel/kgdb.c

···
 	unsigned long ret = 0;
 
 	if (core_kernel_text(addr))
-		ret = addr - MEM_SV_START + PAGE_OFFSET;
+		ret = ktext_writable_addr(addr);
 	else if (is_module_text_address(addr))
 		ret = addr;
 	else

arch/tile/kernel/kprobes.c

···
 	unsigned long addr_wr;
 
 	/* Operate on writable kernel text mapping. */
-	addr_wr = (unsigned long)p->addr - MEM_SV_START + PAGE_OFFSET;
+	addr_wr = ktext_writable_addr(p->addr);
 
 	if (probe_kernel_write((void *)addr_wr, &breakpoint_insn,
 		sizeof(breakpoint_insn)))
···
 	unsigned long addr_wr;
 
 	/* Operate on writable kernel text mapping. */
-	addr_wr = (unsigned long)kp->addr - MEM_SV_START + PAGE_OFFSET;
+	addr_wr = ktext_writable_addr(kp->addr);
 
 	if (probe_kernel_write((void *)addr_wr, &kp->opcode,
 		sizeof(kp->opcode)))