Re: [patch 00/37] cpu/hotplug, x86: Reworked parallel CPU bringup

[Thomas Gleixner]

On Thu, Apr 20 2023 at 10:23, Andrew Cooper wrote:
> On 20/04/2023 9:32 am, Thomas Gleixner wrote:
>> I'm pondering to simply deny parallel mode if x2APIC is not there.
>
> I'm not sure if that will help much.

Spoilsport.

> Just because x2APIC is there doesn't mean it's in use.  There are
> several generations of Intel system which have x2APIC but also use the
> opt-out bit in ACPI tables.  There are some machines which have
> mismatched APIC-ness settings in the BIOS->OS handover.
>
> There's very little you can do on the BSP alone to know for certain that
> the APs come out of wait-for-SIPI already in x2APIC mode.

Yeah. Reading the APIC that early is going to be entertaining too :)

> One way is the ÆPIC Leak "locked into x2APIC mode" giant security
> bodge. 

Bah.

> If the system really does have a CPU with an APIC ID above 0xfe, then
> chances are good that the APs come out consistently...

Anything else would be really magic :)