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

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Message-ID: <54BECDB0.9060709@amd.com>
Date:	Tue, 20 Jan 2015 15:50:40 -0600
From:	Aravind Gopalakrishnan <aravind.gopalakrishnan@....com>
To:	Borislav Petkov <bp@...e.de>
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 1/19/2015 12:21 PM, Borislav Petkov wrote:
> 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.

Right. Just a concern that if the definition of those 5 bits changes 
then it would be a problem.

> 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.
>

Just something from code inspection. So not an immediate issue.

Thanks,
-Aravind.
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