Re: [PATCH] x86/smpboot: Remove 486-isms from the modern AP boot path

From: Andrew Cooper <andrew.cooper3@citrix.com>
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>
Subject: Re: [PATCH] x86/smpboot: Remove 486-isms from the modern AP boot path
Date: Wed, 1 Apr 2020 23:30:10 +0100	[thread overview]
Message-ID: <beefca46-ac7c-374b-e80a-4e7c3af2eb2b@citrix.com> (raw)
In-Reply-To: <alpine.LFD.2.21.2004010001460.3939520@eddie.linux-mips.org>

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