riscv: xip: support runtime trap patching

RISCV_ERRATA_ALTERNATIVE patches text at runtime which is currently
not possible when the kernel is executed from the flash in XIP mode.
Since runtime patching concerns only traps at the moment, let's just
have all the traps reside in RAM anyway if RISCV_ERRATA_ALTERNATIVE
is set. Thus, these functions will be patch-able even when the .text
section is in flash.

Signed-off-by: Vitaly Wool <vitaly.wool@konsulko.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

Author: vwool

arch/riscv/kernel/traps.c

@@ -86,8 +86,13 @@ static void do_trap_error(struct pt_regs *regs, int signo, int code,
 	}
 }
 
+#if defined (CONFIG_XIP_KERNEL) && defined (CONFIG_RISCV_ERRATA_ALTERNATIVE)
+#define __trap_section		__section(".xip.traps")
+#else
+#define __trap_section
+#endif
 #define DO_ERROR_INFO(name, signo, code, str)				\
-asmlinkage __visible void name(struct pt_regs *regs)			\
+asmlinkage __visible __trap_section void name(struct pt_regs *regs)	\
 {									\
 	do_trap_error(regs, signo, code, regs->epc, "Oops - " str);	\
 }
@@ -111,15 +116,15 @@ DO_ERROR_INFO(do_trap_store_misaligned,
 int handle_misaligned_load(struct pt_regs *regs);
 int handle_misaligned_store(struct pt_regs *regs);
 
-asmlinkage void do_trap_load_misaligned(struct pt_regs *regs)
+asmlinkage void __trap_section do_trap_load_misaligned(struct pt_regs *regs)
 {
 	if (!handle_misaligned_load(regs))
 		return;
 	do_trap_error(regs, SIGBUS, BUS_ADRALN, regs->epc,
 		      "Oops - load address misaligned");
 }
 
-asmlinkage void do_trap_store_misaligned(struct pt_regs *regs)
+asmlinkage void __trap_section do_trap_store_misaligned(struct pt_regs *regs)
 {
 	if (!handle_misaligned_store(regs))
 		return;
@@ -146,7 +151,7 @@ static inline unsigned long get_break_insn_length(unsigned long pc)
 	return GET_INSN_LENGTH(insn);
 }
 
-asmlinkage __visible void do_trap_break(struct pt_regs *regs)
+asmlinkage __visible __trap_section void do_trap_break(struct pt_regs *regs)
 {
 #ifdef CONFIG_KPROBES
 	if (kprobe_single_step_handler(regs))

arch/riscv/kernel/vmlinux-xip.lds.S

@@ -99,9 +99,22 @@ SECTIONS
 	}
 	PERCPU_SECTION(L1_CACHE_BYTES)
 
-	. = ALIGN(PAGE_SIZE);
+	. = ALIGN(8);
+	.alternative : {
+		__alt_start = .;
+		*(.alternative)
+		__alt_end = .;
+	}
 	__init_end = .;
 
+	. = ALIGN(16);
+	.xip.traps : {
+		__xip_traps_start = .;
+		*(.xip.traps)
+		__xip_traps_end = .;
+	}
+
+	. = ALIGN(PAGE_SIZE);
 	.sdata : {
 		__global_pointer$ = . + 0x800;
 		*(.sdata*)