Re: [PATCH] SMP initialization: detection and enumeration

[Richard Braun]

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