linux-kernel - Re: [PATCH 4.4 10/24] x86/fpu: Hard-disable lazy FPU mode

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Date:   Tue, 19 Jun 2018 16:02:31 +0100
From:   Ben Hutchings <ben.hutchings@...ethink.co.uk>
To:     Daniel Sangorrin <daniel.sangorrin@...hiba.co.jp>,
        'Greg Kroah-Hartman' <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org
Cc:     stable@...r.kernel.org, 'Andy Lutomirski' <luto@...nel.org>,
        'Rik van Riel' <riel@...hat.com>,
        'Borislav Petkov' <bp@...en8.de>,
        'Brian Gerst' <brgerst@...il.com>,
        'Dave Hansen' <dave.hansen@...ux.intel.com>,
        'Denys Vlasenko' <dvlasenk@...hat.com>,
        'Fenghua Yu' <fenghua.yu@...el.com>,
        "'H. Peter Anvin'" <hpa@...or.com>,
        'Josh Poimboeuf' <jpoimboe@...hat.com>,
        'Linus Torvalds' <torvalds@...ux-foundation.org>,
        'Oleg Nesterov' <oleg@...hat.com>,
        'Peter Zijlstra' <peterz@...radead.org>,
        'Quentin Casasnovas' <quentin.casasnovas@...cle.com>,
        'Thomas Gleixner' <tglx@...utronix.de>, pbonzini@...hat.com,
        'Ingo Molnar' <mingo@...nel.org>
Subject: Re: [PATCH 4.4 10/24] x86/fpu: Hard-disable lazy FPU mode

On Fri, 2018-06-15 at 13:24 +0900, Daniel Sangorrin wrote:
> Hi Greg,
> 
> >  /* Intel-defined CPU features, CPUID level 0x00000001 (ecx), word 4 */
> > --- a/arch/x86/include/asm/fpu/internal.h
> > +++ b/arch/x86/include/asm/fpu/internal.h
> > @@ -58,7 +58,7 @@ extern u64 fpu__get_supported_xfeatures_
> >   */
> >  static __always_inline __pure bool use_eager_fpu(void)
> >  {
> > -	return static_cpu_has_safe(X86_FEATURE_EAGER_FPU);
> > +	return true;
> >  }
> 
> Since this function returns always true then we can remove the code depending on lazy FPU mode.
> Actually this has already been done in "x86/fpu: Remove use_eager_fpu()"
> Ref: https://patchwork.kernel.org/patch/9365883/
>
> >  static void __init fpu__init_parse_early_param(void)
> >  {
> > -	if (cmdline_find_option_bool(boot_command_line, "eagerfpu=off")) {
> > -		eagerfpu = DISABLE;
> > -		fpu__clear_eager_fpu_features();
> > -	}
> 
> Since this patch removes the kernel boot parameter "eagerfpu", maybe we should remove it from the Documentation.
> This has also been done by commit "x86/fpu: Finish excising 'eagerfpu'"
> Ref: https://patchwork.kernel.org/patch/9380673/
> 
> I will try backporting those patches unless anyone has any objections.

This does seem like a good idea—there is quite a bit of dead code left
and it may be hard to backport any further bug fixes in this area
without that removal.

Ben.

-- 
Ben Hutchings, Software Developer                         Codethink Ltd
https://www.codethink.co.uk/                 Dale House, 35 Dale Street
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