[PATCH v2 1/1] riscv: show code leading to exception

4 Sep 2021

To make analyzing exceptions easier output the code that leads to it.
We already do the same on the ARM platform.
Here is an example:
=> exception ebreak
    Unhandled exception: Breakpoint
    EPC: 000000008ff5d50e RA: 000000008ff5d62c TVAL: 0000000000000000
    EPC: 000000008020b50e RA: 000000008020b62c reloc adjusted
Code: 2785 0693 07a0 dce3 fef6 47a5 d563 00e7 (9002)
To disassemble the code we can use the decodecode script:
$ echo 'Code: 2785 0693 07a0 dce3 fef6 47a5 d563 00e7 (9002)' | \
      CROSS_COMPILE=riscv64-linux-gnu- scripts/decodecode
Code: 2785 0693 07a0 dce3 fef6 47a5 d563 00e7 (9002)
    All code
    ========
       0:   2785                    addiw   a5,a5,1
       2:   07a00693                li      a3,122
       6:   fef6dce3                bge     a3,a5,0xfffffffffffffffe
       a:   47a5                    li      a5,9
       c:   00e7d563                bge     a5,a4,0x16
      10:*  9002                    ebreak         <-- trapping instruction
            ...
Code starting with the faulting instruction
    ===========================================
       0:   9002                    ebreak
            ...
As it is not always clear if the first 16 bits are at the start or in the
middle of a 32bit instruction it may become necessary to strip the first
u16 from the output before calling decodecode to get the correct
disassembled code.
Signed-off-by: Heinrich Schuchardt xypron.glpk@gmx.de
---
v2:
    remove support for instructions longer than 32 bit as these are
    not yet specified
---
 arch/riscv/lib/interrupts.c | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/arch/riscv/lib/interrupts.c b/arch/riscv/lib/interrupts.c
index 7525c152b8..100be2e966 100644
--- a/arch/riscv/lib/interrupts.c
+++ b/arch/riscv/lib/interrupts.c
@@ -51,6 +51,38 @@ static void show_regs(struct pt_regs *regs)
 #endif
 }
+/**
+ * instr_len() - get instruction length
+ *
+ * @i:		low 16 bits of the instruction
+ * Return:	number of u16 in instruction
+ */
+static int instr_len(u16 i)
+{
+	if ((i & 0x03) != 0x03)
+		return 1;
+	/* Instructions with more than 32 bits are not yet specified */
+	return 2;
+}
+
+/**
+ * show_code() - display code leading to exception
+ *
+ * @epc:	program counter
+ */
+static void show_code(ulong epc)
+{
+	u16 *pos = (u16 *)(epc & ~1UL);
+	int i, len = instr_len(*pos);
+
+	printf("\nCode: ");
+	for (i = -8; i; ++i)
+		printf("%04x ", pos[i]);
+	printf("(");
+	for (i = 0; i < len; ++i)
+		printf("%04x%s", pos[i], i + 1 == len ? ")\n" : " ");
+}
+
 static void _exit_trap(ulong code, ulong epc, ulong tval, struct pt_regs *regs)
 {
    static const char * const exception_code[] = {
@@ -85,6 +117,7 @@ static void _exit_trap(ulong code, ulong epc, ulong tval, struct pt_regs *regs)
    	       epc - gd->reloc_off, regs->ra - gd->reloc_off);
show_regs(regs);
+	show_code(epc);
    show_efi_loaded_images(epc);
    panic("\n");
 }
--
2.30.2

    