Walt Drummond wrote:
> george anzinger writes:
> > Uh... I do know about this map, but I wonder if it is at all needed.
> > What is the real difference between a logical cpu and the physical one.
> > Or is this only interesting if the machine is not Smp, i.e. all the cpus
> > are not the same? It just seems to me that introducing an additional
> > mapping just slows things down and, if all the cpus are the same, does
> > not really do anything. Of course, I am assuming that ALL usage would
> > be to the logical :)
> Right. That is not always the case. IA32 is somewhat special. ;) The
> logical mapping allows you to, among other things, easily enumerate
> over the set of active processors without having to check if a
> processor exists at the current processor address.
> The difference is apparent when the physical CPU ID is, say, an
> address on a processor bus, or worse, an address on a set of processor
> busses. Take a look at the IA-64's smp.h. The IA64 physical
> processor ID is a 64-bit structure that has to 8-bit ID's; an EID for
> what amounts to a "processor bus" ID and an ID that corresponds to a
> specific processor on a processor bus. Together, they're a system
> global ID for a specific processor. But there is no guarantee that
> the set of global ID's will be contiguous.
> It's possible to have disjoint (non-contiguous) physical processor
> ID's if a processor bus is not completely populated, or there is an
> empty processor slot or odd processor numbering in firmware, or
All that is cool. Still, most places we don't really address the
processor, so the logical cpu number is all we need. Places like
sched_yield, for example, should be using this, not the actual number,
which IMO should only be used when, for some reason, we NEED the hard
address of the cpu. I don't think this ever has to leak out to the
common kernel code, or am i missing something here.