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Subject: Re: [PATCH] x86/smpboot: Remove 486-isms from the modern AP boot path
To:     "Maciej W. Rozycki" <macro@linux-mips.org>,
        Thomas Gleixner <tglx@linutronix.de>
CC:     <hpa@zytor.com>, LKML <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        <x86@kernel.org>, "Jan Kiszka" <jan.kiszka@siemens.com>,
        James Morris <jmorris@namei.org>,
        "David Howells" <dhowells@redhat.com>,
        Matthew Garrett <mjg59@google.com>,
        Josh Boyer <jwboyer@redhat.com>,
        Zhenzhong Duan <zhenzhong.duan@oracle.com>,
        Steve Wahl <steve.wahl@hpe.com>,
        Mike Travis <mike.travis@hpe.com>,
        Dimitri Sivanich <dimitri.sivanich@hpe.com>,
        Arnd Bergmann <arnd@arndb.de>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Giovanni Gherdovich <ggherdovich@suse.cz>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        Len Brown <len.brown@intel.com>,
        Kees Cook <keescook@chromium.org>,
        Martin Molnar <martin.molnar.programming@gmail.com>,
        Pingfan Liu <kernelfans@gmail.com>,
        <jailhouse-dev@googlegroups.com>
References: <20200325101431.12341-1-andrew.cooper3@citrix.com>
 <601E644A-B046-4030-B3BD-280ABF15BF53@zytor.com>
 <87r1xgxzy6.fsf@nanos.tec.linutronix.de>
 <alpine.LFD.2.21.2004010001460.3939520@eddie.linux-mips.org>
From:   Andrew Cooper <andrew.cooper3@citrix.com>
Message-ID: <beefca46-ac7c-374b-e80a-4e7c3af2eb2b@citrix.com>
Date:   Wed, 1 Apr 2020 23:30:10 +0100
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On 01/04/2020 00:35, Maciej W. Rozycki wrote:
> On Wed, 25 Mar 2020, Thomas Gleixner wrote:
>
>>>> @@ -1118,7 +1121,7 @@ static int do_boot_cpu(int apicid, int cpu,
>>>> struct task_struct *idle,
>>>> 		}
>>>> 	}
>>>>
>>>> -	if (x86_platform.legacy.warm_reset) {
>>>> +	if (!APIC_INTEGRATED(boot_cpu_apic_version)) {
>>>> 		/*
>>>> 		 * Cleanup possible dangling ends...
>>>> 		 */
>>> We don't support SMP on 486 and haven't for a very long time. Is there
>>> any reason to retain that code at all?
>> Not that I'm aware off.
>  For the record: this code is for Pentium really, covering original P5 
> systems, which lacked integrated APIC, as well as P54C systems that went 
> beyond dual (e.g. ALR made quad-SMP P54C systems).  They all used external 
> i82489DX APICs for SMP support.  Few were ever manufactured and getting 
> across one let alone running Linux might be tough these days.  I never 
> managed to get one for myself, which would have been helpful for 
> maintaining this code.
>
>  Even though we supported them by spec I believe we never actually ran MP 
> on any 486 SMP system (Alan Cox might be able to straighten me out on 
> this); none that I know of implemented the MPS even though actual hardware 
> might have used the APIC architecture.  Compaq had its competing solution 
> for 486 and newer SMP, actually deployed, the name of which I long forgot.  
> We never supported it due to the lack of documentation combined with the 
> lack of enough incentive for someone to reverse-engineer it.

:)

I chose "486-ism" based on what the MP spec said about external vs
integrated Local APICs.  I can't claim to have any experience of those days.

I guess given v2 of the patch, I guess this should become "Remove
external-LAPIC support from the AP boot path" ?

~Andrew