linux-kernel - Re: [PATCH 01/10] x86/Kconfig: enable X86_X2APIC by default and improve help text

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Date:   Mon, 28 Feb 2022 13:35:56 -0800
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Mateusz Jończyk <mat.jonczyk@...pl>,
        linux-kernel@...r.kernel.org, x86@...nel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, Yinghai Lu <yinghai@...nel.org>
Subject: Re: [PATCH 01/10] x86/Kconfig: enable X86_X2APIC by default and
 improve help text

Hi--

On 2/28/22 13:28, Mateusz Jończyk wrote:
> As many current platforms (most modern Intel CPUs and QEMU) have x2APIC
> present, enable CONFIG_X86_X2APIC by default as it gives performance
> and functionality benefits. Additionally, if the BIOS has already
> switched APIC to x2APIC mode, but CONFIG_X86_X2APIC is disabled, the
> kernel will panic in arch/x86/kernel/apic/apic.c .
> 
> Also improve the help text, which was confusing and really did not
> describe what the feature is about.
> 

> 
> Signed-off-by: Mateusz Jończyk <mat.jonczyk@...pl>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: x86@...nel.org
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: Yinghai Lu <yinghai@...nel.org>
> ---
>  arch/x86/Kconfig | 22 +++++++++++++++++-----
>  1 file changed, 17 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 9f5bd41bf660..64d760a2b81a 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -437,15 +437,27 @@ config X86_FEATURE_NAMES
>  	  If in doubt, say Y.
>  
>  config X86_X2APIC
> -	bool "Support x2apic"
> +	bool "Support CPUs with x2APIC interrupt controllers"
>  	depends on X86_LOCAL_APIC && X86_64 && (IRQ_REMAP || HYPERVISOR_GUEST)
> +	default y
>  	help
> -	  This enables x2apic support on CPUs that have this feature.
> +	  x2APIC is an interrupt controller architecture, a component of which
> +	  (the local APIC) is present in the CPU. It allows faster access to
> +	  the local APIC and supports a larger number of CPUs in the system
> +	  then the predecessors.

	  than

>  
> -	  This allows 32-bit apic IDs (so it can support very large systems),
> -	  and accesses the local apic via MSRs not via mmio.
> +	  x2APIC was introduced in Intel CPUs around 2008 and in AMD EPYC CPUs
> +	  in 2019, but it can be disabled by the BIOS. It is also frequently
> +	  emulated in virtual machines, even when the host CPU does not support
> +	  it. Support in the CPU be checked by executing

	                     CPU can be checked by executing

> +		cat /proc/cpuinfo | grep x2apic
>  
> -	  If you don't know what to do here, say N.
> +	  If this configuration option is disabled, the kernel will not boot on
> +	  some platforms that have x2APIC enabled.
> +
> +	  Say N if you know that your platform does not have x2APIC.
> +
> +	  Otherwise, say Y.
>  
>  config X86_MPPARSE
>  	bool "Enable MPS table" if ACPI

-- 
~Randy