riscv: Introduce alternative mechanism to apply errata solution

Introduce the "alternative" mechanism from ARM64 and x86 to apply the CPU
vendors' errata solution at runtime. The main purpose of this patch is
to provide a framework. Therefore, the implementation is quite basic for
now so that some scenarios could not use this schemei, such as patching
code to a module, relocating the patching code and heterogeneous CPU
topology.

Users could use the macro ALTERNATIVE to apply an errata to the existing
code flow. In the macro ALTERNATIVE, users need to specify the manufacturer
information(vendorid, archid, and impid) for this errata. Therefore, kernel
will know this errata is suitable for which CPU core. During the booting
procedure, kernel will select the errata required by the CPU core and then
patch it. It means that the kernel only applies the errata to the specified
CPU core. In this case, the vendor's errata does not affect each other at
runtime. The above patching procedure only occurs during the booting phase,
so we only take the overhead of the "alternative" mechanism once.

This "alternative" mechanism is enabled by default to ensure that all
required errata will be applied. However, users can disable this feature by
the Kconfig "CONFIG_RISCV_ERRATA_ALTERNATIVE".

Signed-off-by: Vincent Chen <vincent.chen@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

1 files changed, 142 insertions, 0 deletions

diff --git a/arch/riscv/include/asm/alternative-macros.h b/arch/riscv/include/asm/alternative-macros.h
new file mode 100644
index 000000000000..88c08705f64a
--- /dev/null
+++ b/arch/riscv/include/asm/alternative-macros.h
@@ -0,0 +1,142 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __ASM_ALTERNATIVE_MACROS_H
+#define __ASM_ALTERNATIVE_MACROS_H
+
+#ifdef CONFIG_RISCV_ERRATA_ALTERNATIVE
+
+#ifdef __ASSEMBLY__
+
+.macro ALT_ENTRY oldptr newptr vendor_id errata_id new_len
+	RISCV_PTR \oldptr
+	RISCV_PTR \newptr
+	REG_ASM \vendor_id
+	REG_ASM \new_len
+	.word	\errata_id
+.endm
+
+.macro ALT_NEW_CONTENT vendor_id, errata_id, enable = 1, new_c : vararg
+	.if \enable
+	.pushsection .alternative, "a"
+	ALT_ENTRY 886b, 888f, \vendor_id, \errata_id, 889f - 888f
+	.popsection
+	.subsection 1
+888 :
+	\new_c
+889 :
+	.previous
+	.org    . - (889b - 888b) + (887b - 886b)
+	.org    . - (887b - 886b) + (889b - 888b)
+	.endif
+.endm
+
+.macro __ALTERNATIVE_CFG old_c, new_c, vendor_id, errata_id, enable
+886 :
+	\old_c
+887 :
+	ALT_NEW_CONTENT \vendor_id, \errata_id, \enable, \new_c
+.endm
+
+#define _ALTERNATIVE_CFG(old_c, new_c, vendor_id, errata_id, CONFIG_k) \
+	__ALTERNATIVE_CFG old_c, new_c, vendor_id, errata_id, IS_ENABLED(CONFIG_k)
+
+#else /* !__ASSEMBLY__ */
+
+#include <asm/asm.h>
+#include <linux/stringify.h>
+
+#define ALT_ENTRY(oldptr, newptr, vendor_id, errata_id, newlen) \
+	RISCV_PTR " " oldptr "\n" \
+	RISCV_PTR " " newptr "\n" \
+	REG_ASM " " vendor_id "\n" \
+	REG_ASM " " newlen "\n" \
+	".word " errata_id "\n"
+
+#define ALT_NEW_CONSTENT(vendor_id, errata_id, enable, new_c) \
+	".if " __stringify(enable) " == 1\n"				\
+	".pushsection .alternative, \"a\"\n"				\
+	ALT_ENTRY("886b", "888f", __stringify(vendor_id), __stringify(errata_id), "889f - 888f") \
+	".popsection\n"							\
+	".subsection 1\n"						\
+	"888 :\n"							\
+	new_c "\n"							\
+	"889 :\n"							\
+	".previous\n"							\
+	".org	. - (887b - 886b) + (889b - 888b)\n"			\
+	".org	. - (889b - 888b) + (887b - 886b)\n"			\
+	".endif\n"
+
+#define __ALTERNATIVE_CFG(old_c, new_c, vendor_id, errata_id, enable) \
+	"886 :\n"	\
+	old_c "\n"	\
+	"887 :\n"	\
+	ALT_NEW_CONSTENT(vendor_id, errata_id, enable, new_c)
+
+#define _ALTERNATIVE_CFG(old_c, new_c, vendor_id, errata_id, CONFIG_k)	\
+	__ALTERNATIVE_CFG(old_c, new_c, vendor_id, errata_id, IS_ENABLED(CONFIG_k))
+
+#endif /* __ASSEMBLY__ */
+
+#else /* !CONFIG_RISCV_ERRATA_ALTERNATIVE*/
+#ifdef __ASSEMBLY__
+
+.macro __ALTERNATIVE_CFG old_c
+	\old_c
+.endm
+
+#define _ALTERNATIVE_CFG(old_c, new_c, vendor_id, errata_id, CONFIG_k) \
+	__ALTERNATIVE_CFG old_c
+
+#else /* !__ASSEMBLY__ */
+
+#define __ALTERNATIVE_CFG(old_c)  \
+	old_c "\n"
+
+#define _ALTERNATIVE_CFG(old_c, new_c, vendor_id, errata_id, CONFIG_k) \
+	__ALTERNATIVE_CFG(old_c)
+
+#endif /* __ASSEMBLY__ */
+#endif /* CONFIG_RISCV_ERRATA_ALTERNATIVE */
+/*
+ * Usage:
+ *   ALTERNATIVE(old_content, new_content, vendor_id, errata_id, CONFIG_k)
+ * in the assembly code. Otherwise,
+ *   asm(ALTERNATIVE(old_content, new_content, vendor_id, errata_id, CONFIG_k));
+ *
+ * old_content: The old content which is probably replaced with new content.
+ * new_content: The new content.
+ * vendor_id: The CPU vendor ID.
+ * errata_id: The errata ID.
+ * CONFIG_k: The Kconfig of this errata. When Kconfig is disabled, the old
+ *	     content will alwyas be executed.
+ */
+#define ALTERNATIVE(old_content, new_content, vendor_id, errata_id, CONFIG_k) \
+	_ALTERNATIVE_CFG(old_content, new_content, vendor_id, errata_id, CONFIG_k)
+
+/*
+ * A vendor wants to replace an old_content, but another vendor has used
+ * ALTERNATIVE() to patch its customized content at the same location. In
+ * this case, this vendor can create a new macro ALTERNATIVE_2() based
+ * on the following sample code and then replace ALTERNATIVE() with
+ * ALTERNATIVE_2() to append its customized content.
+ *
+ * .macro __ALTERNATIVE_CFG_2 old_c, new_c_1, vendor_id_1, errata_id_1, enable_1, \
+ *                                   new_c_2, vendor_id_2, errata_id_2, enable_2
+ * 886 :
+ *      \old_c
+ * 887 :
+ *      ALT_NEW_CONTENT \vendor_id_1, \errata_id_1, \enable_1, \new_c_1
+ *      ALT_NEW_CONTENT \vendor_id_2, \errata_id_2, \enable_2, \new_c_2
+ * .endm
+ *
+ * #define _ALTERNATIVE_CFG_2(old_c, new_c_1, vendor_id_1, errata_id_1, CONFIG_k_1, \
+ *                                   new_c_2, vendor_id_2, errata_id_2, CONFIG_k_2) \
+ *        __ALTERNATIVE_CFG_2 old_c, new_c_1, vendor_id_1, errata_id_1, IS_ENABLED(CONFIG_k_1), \
+ *                                   new_c_2, vendor_id_2, errata_id_2, IS_ENABLED(CONFIG_k_2) \
+ *
+ * #define ALTERNATIVE_2(old_content, new_content_1, vendor_id_1, errata_id_1, CONFIG_k_1, \
+ *                                    new_content_2, vendor_id_2, errata_id_2, CONFIG_k_2) \
+ *         _ALTERNATIVE_CFG_2(old_content, new_content_1, vendor_id_1, errata_id_1, CONFIG_k_1, \
+ *                                         new_content_2, vendor_id_2, errata_id_2, CONFIG_k_2)
+ *
+ */
+#endif