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

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Message-Id: <4cb980c8-4b96-4ed0-85af-9baa1b7ef30c@app.fastmail.com>
Date: Thu, 11 Jul 2024 09:47:36 +0800
From: "Jiaxun Yang" <jiaxun.yang@...goat.com>
To: "Thomas Bogendoerfer" <tsbogend@...ha.franken.de>
Cc: "Maciej W. Rozycki" <macro@...am.me.uk>,
 "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7月10日七月 下午3:36，Thomas Bogendoerfer写道：
> On Wed, Jul 10, 2024 at 01:34:41PM +0800, Jiaxun Yang wrote:
>> 
>> 
>> 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.
>
> that's just one case, what about NaN2008 binaries on a legacy MIPS CPU ?

Boot tested CIP United's NaN2008 Debian port, works good so far [1].

[1]: http://repo.oss.cipunited.com/debian/README.nan2008.txt

Thanks
- Jiaxun

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

-- 
- Jiaxun