linux-kernel - Re: [PATCH 3/3] x86/irq: update first_system_vector only when X86_LOCAL

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Date:	Sun, 13 Mar 2016 07:35:04 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Jianyu Zhan <nasa4836@...il.com>
cc:	mingo@...hat.com, "H. Peter Anvin" <hpa@...or.com>,
	Aravind.Gopalakrishnan@....com, brgerst@...il.com, bp@...e.de,
	feng.wu@...el.com, jiang.liu@...ux.intel.com,
	Tejun Heo <tj@...nel.org>, dvlasenk@...hat.com,
	penberg@...helsinki.fi, Yinghai Lu <yhlu.kernel@...il.com>,
	andi@...stfloor.org, Andy Lutomirski <luto@...nel.org>,
	ajm@....com, Yinghai Lu <yinghai@...nel.org>,
	Akinobu Mita <akinobu.mita@...il.com>, x86@...nel.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] x86/irq: update first_system_vector only when
 X86_LOCAL_PIC is on

On Sun, 13 Mar 2016, Jianyu Zhan wrote:

> On Sun, Mar 13, 2016 at 4:08 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> > This is pointless, because it's only called when local apic is enabled as all
> > call sites of alloc_intr_gate() depend on CONFIG_X86_LOCAL_APIC ....
> 
> Not exactly,  currently at least  smp_intr_init()  DOES NOT depend on
> CONFIG_X86_LOCAL_APIC:
> 
> static void __init smp_intr_init(void)
> {
> #ifdef CONFIG_SMP

It does, because CONFIG_SMP enables CONFIG_X86_LOCAL_APIC

> I know this is weird, because modern SMP machines implies Local APIC.
> But currently we have CONFIG_SMP detangle from CONFIG_X86_LOCAL_APIC,
> which I think is fine.

Do you actually understand how all that works together?
 
> Another place which is weird is CONFIG_IRQ_WORK.  Technically,  it
> does not depend
> on SMP,   nor even necessary Local APIC.  Actually, it is just a base
> configuration selected
> by others.  But currently we have the

Have you tried to enable it independent from CONFIG_X86_LOCAL_APIC?
 
> >>       i = FIRST_EXTERNAL_VECTOR;
> >> -#ifndef CONFIG_X86_LOCAL_APIC
> >> -#define first_system_vector NR_VECTORS
> >> -#endif
> >>       for_each_clear_bit_from(i, used_vectors, first_system_vector) {
> >
> > And how exactly is this here supposed to compile when CONFIG_X86_LOCAL_APIC=n?
> 
> Dunno.  I guess this code on !CONFIG_X86_LOCAL_APIC case hasn't been
> tested yet ?

It's your job to at least compile test your patches not the job of others.
 
> For CONFIG_X86_LOCAL_APIC case,  the define makes sense.
> But for ! CONFIG_X86_LOCAL_APIC case,  why we confine it to NR_VECTORS
> is a mystery
> to me.  Have digged into git history, but found no proof.

And because it's a mystery you can just change it as you think it's fine and
thereby break the build?
 
> So to maintain consistency, this patch just retain what it is,  but we
> do not bother update it for
> !CONFIG_X86_LOCAL_APIC case.

To maintain consistency we leave it as is, because that actually compiles AND
works.

Thanks,

	tglx