linux-kernel - Re: [PATCH] x86, amd: Set X86_FEATURE_EXTD

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Message-ID: <20150429160658.GC13807@pd.tnic>
Date:	Wed, 29 Apr 2015 18:06:59 +0200
From:	Borislav Petkov <bp@...e.de>
To:	Aravind Gopalakrishnan <Aravind.Gopalakrishnan@....com>
Cc:	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
	x86@...nel.org, hecmargi@....es, dave.hansen@...ux.intel.com,
	mgorman@...e.de, pbonzini@...hat.com, jacob.w.shin@...il.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86, amd: Set X86_FEATURE_EXTD_APICID for future
 processors

On Mon, Apr 27, 2015 at 10:25:51AM -0500, Aravind Gopalakrishnan wrote:
> Decision to use a 4-bit mask or 8-bit mask in default_get_apic_id()
> is controlled by setting capability bit X86_FEATURE_EXTD_APICID.
> 
> Currently, we detect extended APIC ID support by accessing Link
> Transaction Control register D18F0x68 in PCI config space.
> 
> But, not even that is needed as we can safely postulate that future
> AMD processors will support 8-bit APIC IDs and we can simply set that
> feature bit on them, without the PCI access.
> 
> Signed-off-by: Aravind Gopalakrishnan <Aravind.Gopalakrishnan@....com>
> ---
>  arch/x86/kernel/cpu/amd.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
> index fd470eb..20b6813 100644
> --- a/arch/x86/kernel/cpu/amd.c
> +++ b/arch/x86/kernel/cpu/amd.c
> @@ -520,8 +520,16 @@ static void early_init_amd(struct cpuinfo_x86 *c)
>  			set_cpu_cap(c, X86_FEATURE_K6_MTRR);
>  #endif
>  #if defined(CONFIG_X86_LOCAL_APIC) && defined(CONFIG_PCI)
> -	/* check CPU config space for extended APIC ID */
> -	if (cpu_has_apic && c->x86 >= 0xf) {
> +	/*
> +	 * ApicID can always be treated as a 8bit value for
> +	 * AMD APIC versions >= 0x10. So, we can safely set

Expanded the version aspect to:

        /*
         * ApicID can always be treated as a 8bit value for AMD APIC versions
         * >= 0x10 but even old K8s came out of reset with version 0x10. So, we
         * can safely set X86_FEATURE_EXTD_APICID unconditionally for families
         * after 16h.
         */

and applied.

Thanks.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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