target/riscv: Implement the stval/mtval illegal instruction

The stval and mtval registers can optionally contain the faulting
instruction on an illegal instruction exception. This patch adds support
for setting the stval and mtval registers.

The RISC-V spec states that "The stval register can optionally also be
used to return the faulting instruction bits on an illegal instruction
exception...". In this case we are always writing the value on an
illegal instruction.

This doesn't match all CPUs (some CPUs won't write the data), but in
QEMU let's just populate the value on illegal instructions. This won't
break any guest software, but will provide more information to guests.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Message-id: 20211220064916.107241-4-alistair.francis@opensource.wdc.com

target/riscv/cpu.h

@@ -132,6 +132,8 @@ struct CPURISCVState {
     target_ulong frm;
 
     target_ulong badaddr;
+    uint32_t bins;
+
     target_ulong guest_phys_fault_addr;
 
     target_ulong priv_ver;

target/riscv/cpu_helper.c

@@ -1038,6 +1038,9 @@ void riscv_cpu_do_interrupt(CPUState *cs)
             write_gva = true;
             tval = env->badaddr;
             break;
+        case RISCV_EXCP_ILLEGAL_INST:
+            tval = env->bins;
+            break;
         default:
             break;
         }

target/riscv/translate.c

@@ -208,6 +208,9 @@ static void generate_exception_mtval(DisasContext *ctx, int excp)
 
 static void gen_exception_illegal(DisasContext *ctx)
 {
+    tcg_gen_st_i32(tcg_constant_i32(ctx->opcode), cpu_env,
+                   offsetof(CPURISCVState, bins));
+
     generate_exception(ctx, RISCV_EXCP_ILLEGAL_INST);
 }