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

['Greg Kroah-Hartman' <gregkh@linuxfoundation.org>]

On Fri, Jun 15, 2018 at 02:23:08PM +0900, Daniel Sangorrin wrote:
> > -----Original Message-----
> > From: stable-owner@vger.kernel.org [mailto:stable-owner@vger.kernel.org] On
> > Behalf Of 'Greg Kroah-Hartman'
> > Sent: Friday, June 15, 2018 1:56 PM
> > To: Daniel Sangorrin <daniel.sangorrin@toshiba.co.jp>
> > Cc: linux-kernel@vger.kernel.org; stable@vger.kernel.org; 'Andy Lutomirski'
> > <luto@kernel.org>; 'Rik van Riel' <riel@redhat.com>; 'Borislav Petkov'
> > <bp@alien8.de>; 'Brian Gerst' <brgerst@gmail.com>; 'Dave Hansen'
> > <dave.hansen@linux.intel.com>; 'Denys Vlasenko' <dvlasenk@redhat.com>;
> > 'Fenghua Yu' <fenghua.yu@intel.com>; 'H. Peter Anvin' <hpa@zytor.com>; 'Josh
> > Poimboeuf' <jpoimboe@redhat.com>; 'Linus Torvalds'
> > <torvalds@linux-foundation.org>; 'Oleg Nesterov' <oleg@redhat.com>; 'Peter
> > Zijlstra' <peterz@infradead.org>; 'Quentin Casasnovas'
> > <quentin.casasnovas@oracle.com>; 'Thomas Gleixner' <tglx@linutronix.de>;
> > pbonzini@redhat.com; 'Ingo Molnar' <mingo@kernel.org>
> > Subject: Re: [PATCH 4.4 10/24] x86/fpu: Hard-disable lazy FPU mode
> > 
> > On Fri, Jun 15, 2018 at 01:24:27PM +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.
> > 
> > What are the git commit ids of those patches in Linus's tree?  No need
> > to point to patchwork links, I don't use that tool.
> 
> OK, I got it.
> 
> "x86/fpu: Remove use_eager_fpu()": c592b57347069abfc0dcad3b3a302cf882602597
> "x86/fpu: Finish excising 'eagerfpu'": e63650840e8b053aa09ad934877e87e9941ed135

Minor nit.  For kernel commits, the "normal" way we reference them looks
like this:
	c592b5734706 ("x86/fpu: Remove use_eager_fpu()")
	e63650840e8b ("x86/fpu: Finish excising 'eagerfpu'")

Which makes it much easier to read and understand.  That's what we use
for the "Fixes:" tag in commits and in other places (text in commit
messages.)

To automatically generate that format, you can just do:
	git show -s --abbrev-commit --abbrev=12 --pretty=format:"%h (\"%s\")%n"

I recommend just setting up an alias for the above line, otherwise it's
a pain to have to remember how to do it all the time.  Here's what I do:
	$ alias gsr='git show -s --abbrev-commit --abbrev=12 --pretty=format:"%h (\"%s\")%n"'
	$ gsr c592b57347069abfc0dcad3b3a302cf882602597
	c592b5734706 ("x86/fpu: Remove use_eager_fpu()")

> Unfortunately, they don't apply cleanly to stable kernels.

Should be very simple to backport if you want to.  Also I need copies
for the 4.9.y tree as well if you do so.

thanks,

greg k-h