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

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Date:	Fri, 16 Jan 2015 10:59:27 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Thomas Gleixner <tglx@...utronix.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 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.

> +}
> +
>  #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?

>  }
>  
>  #define x2apic_supported()	(cpu_has_x2apic)
> Index: tip/arch/x86/kernel/apic/apic.c
> ===================================================================
> --- tip.orig/arch/x86/kernel/apic/apic.c
> +++ tip/arch/x86/kernel/apic/apic.c
> @@ -1625,10 +1625,7 @@ void __init enable_IR_x2apic(void)
>  	int ret, ir_stat;
>  
>  	if (!IS_ENABLED(CONFIG_X86_X2APIC)) {
> -		u64 msr;
> -
> -		rdmsrl(MSR_IA32_APICBASE, msr);
> -		if (msr & X2APIC_ENABLE)
> +		if (apic_is_x2apic_enabled())
>  			panic("BIOS has enabled x2apic but kernel doesn't support x2apic, please disable x2apic in BIOS.\n");
>  	}
>  
> 
> 
> 

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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