On Wed, May 10, 2023 at 11:33:40AM -0700, Dave Hansen wrote: > 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. Yes, I agree. 0 may become a valid value tomorrow. > > > 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. But you always need an APICID, no? You would only need to get the CPU type only when running on a hybrid CPU; from cpuid_eax(0x0000001a) or any other future mechanism. Yes, I agree that is plausible to have 0 as the CPU type. > > > 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. So maybe changing the function to return -ENOTSUP or -ENODEV on a non-hybrid processor?
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