On Tue, Aug 8, 2023 at 7:10 PM Nikita Shubin <nikita.shu...@maquefel.me> wrote:
>
> From: Nikita Shubin <n.shu...@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.shu...@yadro.com>
Reviewed-by: Alistair Francis <alistair.fran...@wdc.com>
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
>
>
