linux-kernel - Re: [PATCH] x86, cpu, amd: Use 8-bit extractor for NodeId field

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Message-ID: <20150119182142.GF4486@pd.tnic>
Date:	Mon, 19 Jan 2015 19:21:42 +0100
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, riel@...hat.com, mgorman@...e.de, jolsa@...nel.org,
	pbonzini@...hat.com, jacob.w.shin@...il.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86, cpu, amd: Use 8-bit extractor for NodeId field

On Mon, Jan 19, 2015 at 12:18:58PM -0600, Aravind Gopalakrishnan wrote:
> The NodeId field in cpuid_ecx(0x8000001e) is a 8 bit field.
> Although current extractor works fine, it will break if/when
> the most significant 5 bits ever get used.
> 
> So, lets honor the field as architected in the BKDG
> 
> Reference: BKDG for AMD Fam15h Models 00h-0fh,
> CPUID Fn8000_001E_ECX Node Identifiers
> 
> Signed-off-by: Aravind Gopalakrishnan <Aravind.Gopalakrishnan@....com>
> ---
>  arch/x86/kernel/cpu/amd.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
> index a220239..9942b83 100644
> --- a/arch/x86/kernel/cpu/amd.c
> +++ b/arch/x86/kernel/cpu/amd.c
> @@ -300,7 +300,7 @@ static void amd_get_topology(struct cpuinfo_x86 *c)
>  
>  		cpuid(0x8000001e, &eax, &ebx, &ecx, &edx);
>  		nodes = ((ecx >> 8) & 7) + 1;
> -		node_id = ecx & 7;
> +		node_id = ecx & 0xff;

That's {00000b,D18F0x60[NodeId]} and NodeId is 3 bits for enumerating
8 nodes so checking the 3 bits is actually correct as it says that the
other 5 are 0b.

Regardless, is this something hypothetical you've noticed from code
inspection or are we really going to have more than 8 nodes? If the
former, then patch is unnecessary churn.

-- 
Regards/Gruss,
    Boris.

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