Re: [PATCH v3 1/3] target/riscv: Fix tb->flags FS status

[Alistair Francis <alistair23@gmail.com>]

On Wed, Jan 15, 2020 at 4:18 PM <shihpo.hung@sifive.com> wrote:
>
> It was found that running libquantum on riscv-linux qemu produced an
> incorrect result. After investigation, FP registers are not saved
> during context switch due to incorrect mstatus.FS.
>
> In current implementation tb->flags merges all non-disabled state to
> dirty. This means the code in mark_fs_dirty in translate.c that
> handles initial and clean states is unreachable.
>
> This patch fixes it and is successfully tested with:
>   libquantum
>
> Thanks to Richard for pointing out the actual bug.
>
> v3: remove the redundant condition
> v2: root cause FS problem
>
> Suggested-by: Richard Henderson <richard.henderson@linaro.org>
> Signed-off-by: ShihPo Hung <shihpo.hung@sifive.com>
> Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>  target/riscv/cpu.h | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
>
> diff --git a/target/riscv/cpu.h b/target/riscv/cpu.h
> index e59343e..de0a8d8 100644
> --- a/target/riscv/cpu.h
> +++ b/target/riscv/cpu.h
> @@ -293,10 +293,7 @@ static inline void cpu_get_tb_cpu_state(CPURISCVState *env, target_ulong *pc,
>  #ifdef CONFIG_USER_ONLY
>      *flags = TB_FLAGS_MSTATUS_FS;
>  #else
> -    *flags = cpu_mmu_index(env, 0);
> -    if (riscv_cpu_fp_enabled(env)) {
> -        *flags |= TB_FLAGS_MSTATUS_FS;
> -    }
> +    *flags = cpu_mmu_index(env, 0) | (env->mstatus & MSTATUS_FS);

I don't think this is right, you should use the riscv_cpu_fp_enabled() function.

Right now it's the same as env->mstatus & MSTATUS_FS but when the
Hypervisor extension goes in riscv_cpu_fp_enabled() will be more
complex.

Alistair

>  #endif
>  }
>
> --
> 2.7.4
>
>