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

[Richard Henderson]

On 1/14/20 8:28 PM, Alistair Francis wrote:
> On Wed, Jan 15, 2020 at 4:18 PM <address@hidden> 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 <address@hidden>
>> Signed-off-by: ShihPo Hung <address@hidden>
>> Reviewed-by: Richard Henderson <address@hidden>
>> ---
>>  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.

Hmm.  Are you sure something like

  flags |= riscv_cpu_effective_mstatus(env) & MSTATUS_FS;

wouldn't be more appropriate for the hypervisor extension?

I guess I should have another browse through your hv patchset, but I worry now
about bare uses of env->mstatus, if they no longer mean what they appear to 
mean.


r~