On 30 Jul 2020, at 22:06, Richard Braun <rbr...@sceen.net> wrote: > 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.
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). Jess
