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

[Aravind Gopalakrishnan]

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@xxxxxxx>
> > ---
> >   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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