linux-kernel - Re: [patch 41/58] x86/apic: Add max_apic

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Date:   Tue, 18 Jul 2023 20:59:46 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Wei Liu <wei.liu@...nel.org>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Juergen Gross <jgross@...e.com>
Subject: Re: [patch 41/58] x86/apic: Add max_apic_id member

On Tue, Jul 18 2023 at 09:06, Linus Torvalds wrote:
> On Tue, 18 Jul 2023 at 00:47, Thomas Gleixner <tglx@...utronix.de> wrote:
>>
>> The confusing part here is the physical APIC ID vs. the destination
>> mode.
>
> Actually, no, what confused me here ended up being that I didn't see
> any other limit checking at all for the flat mode, and then I was
> "this cannot possibly work up to that limit".
>
> But it turns out that the limit checking appears to be in the
> "physflat" case, not in the simple flat case.
>
> IOW, the physflat probe function says "I'll take it" whenever
> num_possible_cpus() > 8", and that seems to be what then limits the
> flat mode to a max of 8 cpus. So the limit was just in another place
> than I expected.

Right. And obviously you managed to confuse me too :)