On Thu, Jul 30, 2020 at 10:44:40PM +0200, Samuel Thibault wrote: > > > I'm wondering: is it really *that* simple to get the current cpu number, > > > just read a memory location? I'm surprised that this would provide > > > different results on different cpus. > > > > The APIC ID is stored in the Local APIC of each cpu. This address is common > > for > > all Local APIC: accessing this from each cpu, it shows the Local APIC of > > this > > cpu. > > By example, if you access this address from cpu1, you can see the Local > > APIC of > > cpu1. > > So it's a special address whose accesses are trapped within the chip and > don't actually get out on the memory bus?
It's physically memory mapped to the local APIC address space, but because of that, it's also not optimal. All systems I know use a scheme similar to TLS, i.e. using the fs or gs segment register, to fetch a per-CPU structure and from it, per-CPU data. This avoids relying on hardware running at a lower frequency than the CPU. -- Richard Braun
