[PATCH v2] target/riscv: Restore the predicate() NULL check behavior

[Bin Meng]

When reading a non-existent CSR QEMU should raise illegal instruction
exception, but currently it just exits due to the g_assert() check.

This actually reverts commit 0ee342256af9205e7388efdf193a6d8f1ba1a617.
Some comments are also added to indicate that predicate() must be
provided for an implemented CSR.

Reported-by: Fei Wu <fei2.wu@intel.com>
Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>

---

Changes in v2:
- rebase on top of Alistair's riscv-to-apply.next tree

 target/riscv/csr.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/target/riscv/csr.c b/target/riscv/csr.c
index f4d2dcfdc8..7000eb3350 100644
--- a/target/riscv/csr.c
+++ b/target/riscv/csr.c
@@ -3817,6 +3817,11 @@ static inline RISCVException 
riscv_csrrw_check(CPURISCVState *env,
         return RISCV_EXCP_ILLEGAL_INST;
     }
 
+    /* ensure CSR is implemented by checking predicate */
+    if (!csr_ops[csrno].predicate) {
+        return RISCV_EXCP_ILLEGAL_INST;
+    }
+
     /* privileged spec version check */
     if (env->priv_ver < csr_min_priv) {
         return RISCV_EXCP_ILLEGAL_INST;
@@ -3834,7 +3839,6 @@ static inline RISCVException 
riscv_csrrw_check(CPURISCVState *env,
      * illegal instruction exception should be triggered instead of virtual
      * instruction exception. Hence this comes after the read / write check.
      */
-    g_assert(csr_ops[csrno].predicate != NULL);
     RISCVException ret = csr_ops[csrno].predicate(env, csrno);
     if (ret != RISCV_EXCP_NONE) {
         return ret;
@@ -4023,7 +4027,10 @@ static RISCVException write_jvt(CPURISCVState *env, int 
csrno,
     return RISCV_EXCP_NONE;
 }
 
-/* Control and Status Register function table */
+/*
+ * Control and Status Register function table
+ * riscv_csr_operations::predicate() must be provided for an implemented CSR
+ */
 riscv_csr_operations csr_ops[CSR_TABLE_SIZE] = {
     /* User Floating-Point CSRs */
     [CSR_FFLAGS]   = { "fflags",   fs,     read_fflags,  write_fflags },
-- 
2.25.1