linux-kernel - Re: [patch 01/23] x86/apic: Avoid open coded x2apic detection

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Message-ID: <alpine.DEB.2.11.1501221239040.5526@nanos>
Date:	Thu, 22 Jan 2015 12:39:44 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Borislav Petkov <bp@...en8.de>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Jiang Liu <jiang.liu@...ux.intel.com>,
	Joerg Roedel <joro@...tes.org>, x86@...nel.org,
	Tony Luck <tony.luck@...el.com>
Subject: Re: [patch 01/23] x86/apic: Avoid open coded x2apic detection

On Fri, 16 Jan 2015, Borislav Petkov wrote:
> On Thu, Jan 15, 2015 at 09:22:09PM -0000, Thomas Gleixner wrote:
> > enable_IR_x2apic() grew a open coded x2apic detection. Implement a
> > proper helper function which shares the code with the already existing
> > x2apic_enabled().
> > 
> > Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> > ---
> >  arch/x86/include/asm/apic.h |   18 +++++++++---------
> >  arch/x86/kernel/apic/apic.c |    5 +----
> >  2 files changed, 10 insertions(+), 13 deletions(-)
> > 
> > Index: tip/arch/x86/include/asm/apic.h
> > ===================================================================
> > --- tip.orig/arch/x86/include/asm/apic.h
> > +++ tip/arch/x86/include/asm/apic.h
> > @@ -108,6 +108,14 @@ extern u64 native_apic_icr_read(void);
> >  
> >  extern int x2apic_mode;
> >  
> > +static inline bool apic_is_x2apic_enabled(void)
> > +{
> > +	u64 msr;
> > +
> > +	rdmsrl(MSR_IA32_APICBASE, msr);
> 
> Let's do
> 
> 	struct msr m;
> 
> 	if (msr_read(MSR_IA32_APICBASE, &m))
> 		return false;
> 
> 	return m.l & X2APIC_ENABLE;
> 
> so that we do the safe MSR access too. Who knows where this code gets
> executed in the future.

Updated.
 
> > +}
> > +
> >  #ifdef CONFIG_X86_X2APIC
> >  /*
> >   * Make previous memory operations globally visible before
> > @@ -175,15 +183,7 @@ extern void check_x2apic(void);
> >  extern void enable_x2apic(void);
> >  static inline int x2apic_enabled(void)
> >  {
> > -	u64 msr;
> > -
> > -	if (!cpu_has_x2apic)
> > -		return 0;
> > -
> > -	rdmsrl(MSR_IA32_APICBASE, msr);
> > -	if (msr & X2APIC_ENABLE)
> > -		return 1;
> > -	return 0;
> > +	return cpu_has_x2apic && apic_is_x2apic_enabled();
> 
> ... and then there's
> 
> #define x2apic_supported()      (cpu_has_x2apic)
> 
> which does the cpufeature test.
> 
> Can we agree on one interface only and simplify this a bit more?

I think I do that later on, but will make sure.
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