Re: [PATCH v3] MIPS: Implement ieee754 NAN2008 emulation mode

Jiaxun Yang
2024-06-28T00:33:28Z

在2024年6月27日六月 下午8:54，Maciej W. Rozycki写道：
> On Thu, 27 Jun 2024, Jiaxun Yang wrote:
>
>> >> @@ -318,6 +318,10 @@ void mips_set_personality_nan(struct arch_elf_state *state)
>> >> t->thread.fpu.fcr31 = c->fpu_csr31;
>> >> switch (state->nan_2008) {
>> >> case 0:
>> >> + if (!(c->fpu_msk31 & FPU_CSR_NAN2008))
>> >> + t->thread.fpu.fcr31 &= ~FPU_CSR_NAN2008;
>> >> + if (!(c->fpu_msk31 & FPU_CSR_ABS2008))
>> >> + t->thread.fpu.fcr31 &= ~FPU_CSR_ABS2008;
>> >
>> > why is this needed?
>>
>> Because t->thread.fpu.fcr31 comes from c->fpu_csr31, in this case we the default
>> value of c->fpu_csr31 is read from hardware and we don't know what would that be.
>
> But it has always been like this. What has changed with your patch that
> you need to mask the bit out now?

After this patch kernel's copy of t->thread.fpu.fcr31 can disagree with hardware.
When disagree happens, we trigger emulation.

Before that patch for nan legacy binary running on nan2008 CPU t->thread.fpu.fcr31
will still be nan2008 (for ieee754=relaxed) so that's not relevant.

Thanks
>
> Maciej

--
- Jiaxun