target/riscv: Add some comments to clarify the priority policy of riscv_csrrw_check()

The priority policy of riscv_csrrw_check() was once adjusted in
commit eacaf4401956 ("target/riscv: Fix priority of csr related check in riscv_csrrw_check")
whose commit message says the CSR existence check should come before
the access control check, but the code changes did not agree with
the commit message, that the predicate() check actually came after
the read / write check.

In fact this was intentional. Add some comments there so that people
won't bother trying to change it without a solid reason.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Weiwei Li<liweiwei@iscas.ac.cn>
Message-ID: <20230228104035.1879882-3-bmeng@tinylab.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

1 files changed, 10 insertions, 1 deletions

diff --git a/target/riscv/csr.c b/target/riscv/csr.c
index 75a540b..4cc2c63 100644
--- a/target/riscv/csr.c
+++ b/target/riscv/csr.c
@@ -3776,11 +3776,12 @@ static inline RISCVException riscv_csrrw_check(CPURISCVState *env,
     int read_only = get_field(csrno, 0xC00) == 3;
     int csr_min_priv = csr_ops[csrno].min_priv_ver;
 
-    /* ensure the CSR extension is enabled. */
+    /* ensure the CSR extension is enabled */
     if (!cpu->cfg.ext_icsr) {
         return RISCV_EXCP_ILLEGAL_INST;
     }
 
+    /* privileged spec version check */
     if (env->priv_ver < csr_min_priv) {
         return RISCV_EXCP_ILLEGAL_INST;
     }
@@ -3790,10 +3791,18 @@ static inline RISCVException riscv_csrrw_check(CPURISCVState *env,
         return RISCV_EXCP_ILLEGAL_INST;
     }
 
+    /* read / write check */
     if (write_mask && read_only) {
         return RISCV_EXCP_ILLEGAL_INST;
     }
 
+    /*
+     * The predicate() not only does existence check but also does some
+     * access control check which triggers for example virtual instruction
+     * exception in some cases. When writing read-only CSRs in those cases
+     * illegal instruction exception should be triggered instead of virtual
+     * instruction exception. Hence this comes after the read / write check.
+     */
     RISCVException ret = csr_ops[csrno].predicate(env, csrno);
     if (ret != RISCV_EXCP_NONE) {
         return ret;