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

Aravind Gopalakrishnan
2015-01-20T21:50:57Z

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@amd.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.