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

Jiaxun Yang
2024-07-10T05:35:05Z

在2024年7月9日七月 下午4:55，Thomas Bogendoerfer写道：
> On Fri, Jun 28, 2024 at 01:33:06AM +0100, Jiaxun Yang wrote:
>>
>>
>> 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.
>
> I'm considering to apply your patch, how much testing/verification did
> this patch see ? Do have some test binaries ?

It has been tested against Debian rootfs. There is no need to test againt special binary,
but you need NaN2008 hardware such as Loongson 3A4000.

Thanks
- Jiaxun
>
> Thomas.
>
> --
> Crap can work. Given enough thrust pigs will fly, but it's not necessarily a
> good idea. [ RFC1925, 2.3 ]

--
- Jiaxun