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

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Message-Id: <fbd421a6-cf37-49ab-bdbe-6128a7cae8be@app.fastmail.com>
Date: Fri, 28 Jun 2024 01:33:06 +0100
From: "Jiaxun Yang" <jiaxun.yang@...goat.com>
To: "Maciej W. Rozycki" <macro@...am.me.uk>
Cc: "Thomas Bogendoerfer" <tsbogend@...ha.franken.de>,
 "Jonathan Corbet" <corbet@....net>, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org,
 "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
 Philippe Mathieu-Daudé <philmd@...aro.org>
Subject: Re: [PATCH v3] MIPS: Implement ieee754 NAN2008 emulation mode

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