Messages in this thread | | | Date | Wed, 10 May 2023 11:18:15 -0700 | From | Ricardo Neri <> | Subject | Re: [tip: perf/core] x86/cpu: Add helper function to get the type of the current hybrid CPU |
| |
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.
| |