powerpc/asm: Add a patch_site macro & helpers for patching instructions

BugLink: https://bugs.launchpad.net/bugs/1830176

commit 06d0bbc6 upstream.

Add a macro and some helper C functions for patching single asm
instructions.

The gas macro means we can do something like:

  1:	nop
  	patch_site 1b, patch__foo

Which is less visually distracting than defining a GLOBAL symbol at 1,
and also doesn't pollute the symbol table which can confuse eg. perf.

These are obviously similar to our existing feature sections, but are
not automatically patched based on CPU/MMU features, rather they are
designed to be manually patched by C code at some arbitrary point.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

arch/powerpc/include/asm/code-patching-asm.h

+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * Copyright 2018, Michael Ellerman, IBM Corporation.
+ */
+#ifndef _ASM_POWERPC_CODE_PATCHING_ASM_H
+#define _ASM_POWERPC_CODE_PATCHING_ASM_H
+
+/* Define a "site" that can be patched */
+.macro patch_site label name
+	.pushsection ".rodata"
+	.balign 4
+	.global \name
+\name:
+	.4byte	\label - .
+	.popsection
+.endm
+
+#endif /* _ASM_POWERPC_CODE_PATCHING_ASM_H */

arch/powerpc/include/asm/code-patching.h

@@ -28,6 +28,8 @@ unsigned int create_cond_branch(const unsigned int *addr,
 				unsigned long target, int flags);
 int patch_branch(unsigned int *addr, unsigned long target, int flags);
 int patch_instruction(unsigned int *addr, unsigned int instr);
+int patch_instruction_site(s32 *addr, unsigned int instr);
+int patch_branch_site(s32 *site, unsigned long target, int flags);
 
 int instr_is_relative_branch(unsigned int instr);
 int instr_is_branch_to_addr(const unsigned int *instr, unsigned long addr);

arch/powerpc/lib/code-patching.c

@@ -32,6 +32,22 @@ int patch_branch(unsigned int *addr, unsigned long target, int flags)
 	return patch_instruction(addr, create_branch(addr, target, flags));
 }
 
+int patch_branch_site(s32 *site, unsigned long target, int flags)
+{
+	unsigned int *addr;
+
+	addr = (unsigned int *)((unsigned long)site + *site);
+	return patch_instruction(addr, create_branch(addr, target, flags));
+}
+
+int patch_instruction_site(s32 *site, unsigned int instr)
+{
+	unsigned int *addr;
+
+	addr = (unsigned int *)((unsigned long)site + *site);
+	return patch_instruction(addr, instr);
+}
+
 unsigned int create_branch(const unsigned int *addr,
 			   unsigned long target, int flags)
 {