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

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Message-ID: <Zo457UgAkhbAgm2R@alpha.franken.de>
Date: Wed, 10 Jul 2024 09:36:13 +0200
From: Thomas Bogendoerfer <tsbogend@...ha.franken.de>
To: Jiaxun Yang <jiaxun.yang@...goat.com>
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

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 ?

Thomas.

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