Re: [PATCH v7 6/9] x86/smpboot: Support parallel startup of secondary CPUs

[Thomas Gleixner]

On Wed, Feb 08 2023 at 00:09, Brian Gerst wrote:
> On Tue, Feb 7, 2023 at 6:10 PM Usama Arif <usama.arif@xxxxxxxxxxxxx> wrote:
>> Introduce a global variable 'smpboot_control' indicating to the AP how
>> it should find its APIC ID. For a serialized bringup, the APIC ID is
>> explicitly passed in the low bits of smpboot_control, while for parallel
>> mode there are flags directing the AP to find its APIC ID in CPUID leaf
>> 0x0b (for X2APIC mode) or CPUID leaf 0x01 where 8 bits are sufficient.
>
> For the serialized bringup case, it would be simpler to just put the
> cpu number in the lower bits instead of the APIC id, skipping the
> lookup.

Yes and no. It can be done, but I rather prefer the consistency of the
data and the mechanism. The "overhead" or "win" is not even measurable.

Thanks,

        tglx