Re: [PULL 46/49] target/riscv: Don't assume PMU counters are continuous

[Peter Maydell]

On Tue, 7 Nov 2023 at 02:36, Alistair Francis <alistair23@gmail.com> wrote:
>
> From: Rob Bradford <rbradford@rivosinc.com>
>
> Check the PMU available bitmask when checking if a counter is valid
> rather than comparing the index against the number of PMUs.
>
> Signed-off-by: Rob Bradford <rbradford@rivosinc.com>
> Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
> Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
> Reviewed-by: Atish Patra <atishp@rivosinc.com>
> Message-ID: <20231031154000.18134-3-rbradford@rivosinc.com>
> Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
> ---
>  target/riscv/csr.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/target/riscv/csr.c b/target/riscv/csr.c
> index fc26b52c88..fde7ce1a53 100644
> --- a/target/riscv/csr.c
> +++ b/target/riscv/csr.c
> @@ -188,7 +188,8 @@ static RISCVException zcmt(CPURISCVState *env, int csrno)
>  #if !defined(CONFIG_USER_ONLY)
>  static RISCVException mctr(CPURISCVState *env, int csrno)
>  {
> -    int pmu_num = riscv_cpu_cfg(env)->pmu_num;
> +    RISCVCPU *cpu = env_archcpu(env);
> +    uint32_t pmu_avail_ctrs = cpu->pmu_avail_ctrs;
>      int ctr_index;
>      int base_csrno = CSR_MHPMCOUNTER3;
>
> @@ -197,7 +198,7 @@ static RISCVException mctr(CPURISCVState *env, int csrno)
>          base_csrno += 0x80;
>      }
>      ctr_index = csrno - base_csrno;
> -    if (!pmu_num || ctr_index >= pmu_num) {
> +    if ((BIT(ctr_index) & pmu_avail_ctrs >> 3) == 0) {
>          /* The PMU is not enabled or counter is out of range */
>          return RISCV_EXCP_ILLEGAL_INST;
>      }

Hi; Coverity is not convinced that ctr_index is necessarily
guaranteed to be within the valid range to be an argument
to BIT() (eg that it won't be negative). Looking at the
code as a human I'm pretty unsure too. Could somebody have
a look at this and maybe improve the readability / add an
assertion / fix a bug if any ? (CID 1523910)

More generally there are about half a dozen other riscv
issues in Coverity at the moment, so if somebody who knows
the riscv code could have a look at them that would be great.

thanks
-- PMM