On Fri, May 05, 2023 at 09:42:36AM -0700, Dave Hansen wrote:
> On 4/20/21 03:46, tip-bot2 for Ricardo Neri wrote:
> > +#define X86_HYBRID_CPU_TYPE_ID_SHIFT 24
> > +
> > +/**
> > + * get_this_hybrid_cpu_type() - Get the type of this hybrid CPU
> > + *
> > + * Returns the CPU type [31:24] (i.e., Atom or Core) of a CPU in
> > + * a hybrid processor. If the processor is not hybrid, returns 0.
> > + */
> > +u8 get_this_hybrid_cpu_type(void)
> > +{
> > + if (!cpu_feature_enabled(X86_FEATURE_HYBRID_CPU))
> > + return 0;
> > +
> > + return cpuid_eax(0x0000001a) >> X86_HYBRID_CPU_TYPE_ID_SHIFT;
> > +}
>
> Hi Folks,
Hi Dave,
>
> Sorry to dredge up an old thread. But, where does this information
> about "If the processor is not hybrid, returns 0." come from?
>
> What is there to keep cpuid_eax(0x0000001a) from having 0x0 in those
> bits? Seems to me like 0 is theoretically a valid hybrid CPU type. Right?
My reasoning was that according to the Intel SDM the only valid values were
0x20 and 0x40. 0 was meant to be an invalid value.
I read the SDM again. It seems that cpuid_eax(0x0000001a) already returns
0 when the leaf does not exist.
Probably the check for X86_FEATURE_HYBRID_CPU is not needed.
Still, callers need to check for a valid value, IMO.
|