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Date: Sun, 18 Jan 2009 15:25:54 -0600
From: Jack Steiner <steiner@sgi.com>
To: Ingo Molnar <mingo@elte.hu>
Cc: Mike Travis <travis@sgi.com>, LKML <linux-kernel@vger.kernel.org>,
       "H. Peter Anvin" <hpa@zytor.com>, Thomas Gleixner <tglx@linutronix.de>,
       Yinghai Lu <yinghai@kernel.org>,
       "Maciej W. Rozycki" <macro@linux-mips.org>
Subject: Re: [PATCH] x86: put trigger in to detect mismatched apic versions.
Message-ID: <20090118212553.GA164127@sgi.com>
References: <49704DF6.8040205@sgi.com> <20090116092540.GC4305@elte.hu> <4970C984.30202@sgi.com> <20090116223011.GB3899@elte.hu> <497116A8.5080900@sgi.com> <20090117030752.GB127262@sgi.com> <20090118190849.GB858@elte.hu>
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On Sun, Jan 18, 2009 at 08:08:49PM +0100, Ingo Molnar wrote:
> 
> * Jack Steiner <steiner@sgi.com> wrote:
> 
> > > Btw, I checked with our UV architect and the problem is that we need a 
> > > 16 bit apic id which is what caused the MAX_APICS to be bumped to 32k. 
> > > The lower 8 bits are the normal apic id, and the upper bit relate to 
> > > the node.  This means cpu 0 on node 0 has the same apic id as cpu 0 on 
> > > node 1, etc.  I also asked about whether we could rely on always 
> > > having
> > 
> > Not strictly true. The apicids in the ACPI tables are always globally 
> > unique across the entire system. Because of the size of UV systems, UV 
> > needs 16 bit apicids. This fits in the ACPI apicid id/eid fields.
> > 
> > The actual processor apicid register is unfortunately only 11 bits and 
> > there are some restrictions on the actual values loaded into the apicid 
> > register.
> > 
> > If we can put unique ids into the apicid register, we do. If we can't, 
> > the function that reads the apicid will automatically supply the rest of 
> > the bits.  Most of the kernel is unaware that the processor apicid 
> > register may have only a subset of the bits that are in the ACPI tables.
> 
> apicid remapping is something we need/want, so we cannot remove that 
> array. But it would be nice to offload such properties to the percpu area 
> instead - is there any reason why that is hard? The local apic is attached 
> to a CPU in any case. Is there some early init reason that complicates 
> this?

I can't think of any reason why it could not be moved into
the percpu data area. Mike???

--- jack