riscv: integrate alternatives better into the main architecture

Right now the alternatives need to be explicitly enabled and
erratas are limited to SiFive ones.

We want to use alternatives not only for patching soc erratas,
but in the future also for handling different behaviour depending
on the existence of future extensions.

So move the core alternatives over to the kernel subdirectory
and move the CONFIG_RISCV_ALTERNATIVE to be a hidden symbol
which we expect relevant erratas and extensions to just select
if needed.

Signed-off-by: Heiko Stuebner <[e-mail]>
Reviewed-by: Philipp Tomsich <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Palmer Dabbelt <[e-mail]>

arch/riscv/Kconfig

···
 	  Specify the maximum number of NUMA Nodes available on the target
 	  system.  Increases memory reserved to accommodate various tables.
 
+config RISCV_ALTERNATIVE
+	bool
+	depends on !XIP_KERNEL
+	help
+	  This Kconfig allows the kernel to automatically patch the
+	  errata required by the execution platform at run time. The
+	  code patching is performed once in the boot stages. It means
+	  that the overhead from this mechanism is just taken once.
+
 config RISCV_ISA_C
 	bool "Emit compressed instructions when building Linux"
 	default y

arch/riscv/Kconfig.erratas

···
 menu "CPU errata selection"
 
-config RISCV_ERRATA_ALTERNATIVE
-	bool "RISC-V alternative scheme"
-	depends on !XIP_KERNEL
-	default y
-	help
-	  This Kconfig allows the kernel to automatically patch the
-	  errata required by the execution platform at run time. The
-	  code patching is performed once in the boot stages. It means
-	  that the overhead from this mechanism is just taken once.
-
 config ERRATA_SIFIVE
 	bool "SiFive errata"
-	depends on RISCV_ERRATA_ALTERNATIVE
+	depends on !XIP_KERNEL
+	select RISCV_ALTERNATIVE
 	help
 	  All SiFive errata Kconfig depend on this Kconfig. Disabling
 	  this Kconfig will disable all SiFive errata. Please say "Y"

arch/riscv/Kconfig.socs

···
 	select CLK_SIFIVE
 	select CLK_SIFIVE_PRCI
 	select SIFIVE_PLIC
-	select RISCV_ERRATA_ALTERNATIVE if !XIP_KERNEL
 	select ERRATA_SIFIVE if !XIP_KERNEL
 	help
 	  This enables support for SiFive SoC platform hardware.

arch/riscv/Makefile

···
 
 head-y := arch/riscv/kernel/head.o
 
-core-$(CONFIG_RISCV_ERRATA_ALTERNATIVE) += arch/riscv/errata/
+core-y += arch/riscv/errata/
 core-$(CONFIG_KVM) += arch/riscv/kvm/
 
 libs-y += arch/riscv/lib/

arch/riscv/errata/Makefile

···
-obj-y	+= alternative.o
 obj-$(CONFIG_ERRATA_SIFIVE) += sifive/

arch/riscv/include/asm/alternative-macros.h

···
 #ifndef __ASM_ALTERNATIVE_MACROS_H
 #define __ASM_ALTERNATIVE_MACROS_H
 
-#ifdef CONFIG_RISCV_ERRATA_ALTERNATIVE
+#ifdef CONFIG_RISCV_ALTERNATIVE
 
 #ifdef __ASSEMBLY__
 
···
 
 #endif /* __ASSEMBLY__ */
 
-#else /* !CONFIG_RISCV_ERRATA_ALTERNATIVE*/
+#else /* CONFIG_RISCV_ALTERNATIVE */
 #ifdef __ASSEMBLY__
 
 .macro __ALTERNATIVE_CFG old_c
···
 	__ALTERNATIVE_CFG(old_c)
 
 #endif /* __ASSEMBLY__ */
-#endif /* CONFIG_RISCV_ERRATA_ALTERNATIVE */
+#endif /* CONFIG_RISCV_ALTERNATIVE */
+
 /*
  * Usage:
  *   ALTERNATIVE(old_content, new_content, vendor_id, errata_id, CONFIG_k)

arch/riscv/include/asm/alternative.h

···
 
 #ifndef __ASSEMBLY__
 
+#ifdef CONFIG_RISCV_ALTERNATIVE
+
 #include <linux/init.h>
 #include <linux/types.h>
 #include <linux/stddef.h>
···
 
 void sifive_errata_patch_func(struct alt_entry *begin, struct alt_entry *end,
 			      unsigned long archid, unsigned long impid);
+
+#else /* CONFIG_RISCV_ALTERNATIVE */
+
+static inline void apply_boot_alternatives(void) { }
+
+#endif /* CONFIG_RISCV_ALTERNATIVE */
 
 #endif
 #endif

arch/riscv/kernel/Makefile

···
 extra-y += vmlinux.lds
 
 obj-y	+= soc.o
+obj-$(CONFIG_RISCV_ALTERNATIVE) += alternative.o
 obj-y	+= cpu.o
 obj-y	+= cpufeature.o
 obj-y	+= entry.o

arch/riscv/kernel/smpboot.c

···
 void __init smp_prepare_boot_cpu(void)
 {
 	init_cpu_topology();
-#ifdef CONFIG_RISCV_ERRATA_ALTERNATIVE
 	apply_boot_alternatives();
-#endif
 }
 
 void __init smp_prepare_cpus(unsigned int max_cpus)

arch/riscv/kernel/traps.c

···
 	}
 }
 
-#if defined (CONFIG_XIP_KERNEL) && defined (CONFIG_RISCV_ERRATA_ALTERNATIVE)
+#if defined(CONFIG_XIP_KERNEL) && defined(CONFIG_RISCV_ALTERNATIVE)
 #define __trap_section		__section(".xip.traps")
 #else
 #define __trap_section