linux-kernel - Re: [PATCH] x86: add apic probe for genapic 64bit v2

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Message-ID: <20080722181056.GD14380@linux-os.sc.intel.com>
Date:	Tue, 22 Jul 2008 11:10:56 -0700
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	Yinghai Lu <yhlu.kernel@...il.com>
Cc:	"Siddha, Suresh B" <suresh.b.siddha@...el.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, Jack Steiner <steiner@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: add apic probe for genapic 64bit v2

On Tue, Jul 22, 2008 at 11:05:17AM -0700, Yinghai Lu wrote:
> On Tue, Jul 22, 2008 at 10:57 AM, Suresh Siddha
> <suresh.b.siddha@...el.com> wrote:
> > Yinghai, though not directly realted to this probe cleanup, what is this
> > 'max_physical_apicid' doing.
> >
> > I don't understand the need for this commit.
> >
> >> commit e0da33646826b66ef933d47ea2fb7a693fd849bf
> >> Author: Yinghai Lu <yhlu.kernel@...il.com>
> >> Date:   Sun Jun 8 18:29:22 2008 -0700
> >>
> >>     x86: introduce max_physical_apicid for bigsmp switching
> >>
> >>     a multi-socket test-system with 3 or 4 ioapics, when 4 dualcore cpus or
> >>     2 quadcore cpus installed, needs to switch to bigsmp or physflat.
> >>
> >>     CPU apic id is [4,11] instead of [0,7], and we need to check max apic
> >>     id instead of cpu numbers.
> >
> > for logical flat, it really shouldn't matter how large the physical apic id
> > is. In logical flat, OS programs the LDR and the limitation is the
> > number of cpu's which is '8'.
> 
> before i clean up 32bit, 64bit already used max_physical_apicid to
> check if need to switch to physflat.

No. It was introduced by your commit e0da33646826b66ef933d47ea2fb7a693fd849bf

we were using num_possible_cpus > 8 check before.

> for 32bit, if not switch to bigsmp, cpu with physical apic id >=8 can
> not be started by BSP.

why?

thanks,
suresh
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