Re: [PATCH] SMP initialization: detection and enumeration

[Almudena Garcia]

> You can build a CPU ID table on the BSP, and associate CPU IDs
> with all local APIC IDs discovered, and use that look-up table later

> to obtain a CPU ID from the local APIC ID of the CPU running the code.

It's my idea, of course. I don't know what you refer to exactly with "using a segment register". How Does It Work?

Maybe, in next versions, I can improve the implementation using that.

El jue., 30 jul. 2020 a las 23:12, Richard Braun (<rbraun@sceen.net>) escribió:

On Thu, Jul 30, 2020 at 10:09:01PM +0100, Jessica Clarke wrote:
> > 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.
>
> You need to do that anyway if you want any guarantees over _what_ the
> IDs are (normally you want 0 for the BSP, 1 to N-1 for the APs).

Not really. You can build a CPU ID table on the BSP, and associate CPU IDs
with all local APIC IDs discovered, and use that look-up table later
to obtain a CPU ID from the local APIC ID of the CPU running the code.
It's just too slow compared to using a segment register.

--
Richard Braun