Re: [PATCH v3] target/riscv: don't read CSR in riscv_csrrw_do64

[Alistair Francis]

On Tue, Aug 8, 2023 at 7:10 PM Nikita Shubin <nikita.shubin@maquefel.me> wrote:
>
> From: Nikita Shubin <n.shubin@yadro.com>
>
> As per ISA:
>
> "For CSRRWI, if rd=x0, then the instruction shall not read the CSR and
> shall not cause any of the side effects that might occur on a CSR read."
>
> trans_csrrwi() and trans_csrrw() call do_csrw() if rd=x0, do_csrw() calls
> riscv_csrrw_do64(), via helper_csrw() passing NULL as *ret_value.
>
> Signed-off-by: Nikita Shubin <n.shubin@yadro.com>

Thanks!

Applied to riscv-to-apply.next

Alistair

> ---
> Changelog v2:
> - fixed uninitialized old_value
>
> Changelog v3:
> - reword comment and commit message as Deniel suggested
>
> ---
>  target/riscv/csr.c | 24 +++++++++++++++---------
>  1 file changed, 15 insertions(+), 9 deletions(-)
>
> diff --git a/target/riscv/csr.c b/target/riscv/csr.c
> index ea7585329e..c5564d6d53 100644
> --- a/target/riscv/csr.c
> +++ b/target/riscv/csr.c
> @@ -3908,21 +3908,27 @@ static RISCVException riscv_csrrw_do64(CPURISCVState 
> *env, int csrno,
>                                         target_ulong write_mask)
>  {
>      RISCVException ret;
> -    target_ulong old_value;
> +    target_ulong old_value = 0;
>
>      /* execute combined read/write operation if it exists */
>      if (csr_ops[csrno].op) {
>          return csr_ops[csrno].op(env, csrno, ret_value, new_value, 
> write_mask);
>      }
>
> -    /* if no accessor exists then return failure */
> -    if (!csr_ops[csrno].read) {
> -        return RISCV_EXCP_ILLEGAL_INST;
> -    }
> -    /* read old value */
> -    ret = csr_ops[csrno].read(env, csrno, &old_value);
> -    if (ret != RISCV_EXCP_NONE) {
> -        return ret;
> +    /*
> +     * ret_value == NULL means that rd=x0 and we're coming from helper_csrw()
> +     * and we can't throw side effects caused by CSR reads.
> +     */
> +    if (ret_value) {
> +        /* if no accessor exists then return failure */
> +        if (!csr_ops[csrno].read) {
> +            return RISCV_EXCP_ILLEGAL_INST;
> +        }
> +        /* read old value */
> +        ret = csr_ops[csrno].read(env, csrno, &old_value);
> +        if (ret != RISCV_EXCP_NONE) {
> +            return ret;
> +        }
>      }
>
>      /* write value if writable and write mask set, otherwise drop writes */
> --
> 2.39.2
>
>