On 5/10/23 11:18, Ricardo Neri wrote: >> 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. That doesn't make any sense to me really. Just because today's SDM doesn't have a value doesn't mean that it becomes an invalid value tomorrow. For instance, there's no model 0xEE today. But that doesn't make it *INVALID*, it just means there's not one defined *today*. Today's kernel shouldn't fall over if it runs on an model==0xEE system. > I read the SDM again. It seems that cpuid_eax(0x0000001a) already returns > 0 when the leaf does not exist. Right, but this isn't really relevant here either. A CPU's APICID comes out of a leaf that can be unsupported (not exist). That doesn't make APICID==0 invalid in any way. > Probably the check for X86_FEATURE_HYBRID_CPU is not needed. > > Still, callers need to check for a valid value, IMO. Right. But if they're just going to check the number that comes back from this function, 0 can't represent an invalid value.
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