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Re: [PATCH 02/18] target/riscv: Correct the priority policy of riscv_csr

From: weiwei
Subject: Re: [PATCH 02/18] target/riscv: Correct the priority policy of riscv_csrrw_check()
Date: Tue, 14 Feb 2023 16:43:26 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.7.1

On 2023/2/14 02:01, Bin Meng wrote:
The priority policy of riscv_csrrw_check() was once adjusted in
commit eacaf4401956 ("target/riscv: Fix priority of csr related check in 
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 came
after the read / write check.
Hi Bin Meng,

Let me explain why I put read-only check before predicate() check in commit eacaf4401956:

*  The predicates don't do existence check only. They also do some access-control check , and

    will trigger virtual instruction exception in come cases.  I think read-only check should be done

    before these access-control check, and trigger illegal instruction exception instead of virtual

   instruction exception when writing to read-only CSRs in these cases.

 *  Read-only  check will trigger ILLEGAL_INST exception which is  also the exception triggered when

     CSR is not existed, so put this check before existence check will not affect the final exception.


Weiwei Li

Fixes: eacaf4401956 ("target/riscv: Fix priority of csr related check in 
Signed-off-by: Bin Meng <bmeng@tinylab.org>

  target/riscv/csr.c | 8 ++++----
  1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/target/riscv/csr.c b/target/riscv/csr.c
index 1b0a0c1693..c2dd9d5af0 100644
--- a/target/riscv/csr.c
+++ b/target/riscv/csr.c
@@ -3793,15 +3793,15 @@ static inline RISCVException 
riscv_csrrw_check(CPURISCVState *env,
          return RISCV_EXCP_ILLEGAL_INST;
- if (write_mask && read_only) {
-        return RISCV_EXCP_ILLEGAL_INST;
-    }
      RISCVException ret = csr_ops[csrno].predicate(env, csrno);
      if (ret != RISCV_EXCP_NONE) {
          return ret;
+ if (write_mask && read_only) {
+        return RISCV_EXCP_ILLEGAL_INST;
+    }
  #if !defined(CONFIG_USER_ONLY)
      int csr_priv, effective_priv = env->priv;

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