linux-kernel - Re: [patch 26/26] x64, x2apic/intr-remap: introduce CONFIG_INTR

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Date:	Thu, 10 Jul 2008 18:50:59 -0700
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	Yong Wang <yong.y.wang@...ux.intel.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"Siddha, Suresh B" <suresh.b.siddha@...el.com>,
	"mingo@...e.hu" <mingo@...e.hu>, "hpa@...or.com" <hpa@...or.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"arjan@...ux.intel.com" <arjan@...ux.intel.com>,
	"andi@...stfloor.org" <andi@...stfloor.org>,
	"jbarnes@...tuousgeek.org" <jbarnes@...tuousgeek.org>,
	"steiner@....com" <steiner@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [patch 26/26] x64, x2apic/intr-remap: introduce CONFIG_INTR_REMAP

On Thu, Jul 10, 2008 at 04:37:33PM -0700, Yong Wang wrote:
> > Is it possible to run in x2apic mode without running in interrupt remapping mode?
> > Just send the irqs directly to the cpu?  It feels like it should be if an iommu
> > doesn't get in the way.
> >
> 
> Yes, it is possible to run in x2apic mode without intr-remap. However, the extended
> cpu addressability will not be fully utilized that way.

No. CPU can't be in x2apic mode with out enabling interrupt-remapping.
Interrupts may not work (even when we have few cpu's). For example, in logical
cluster mode, logical x2apic id (initialized by HW) will be > 16 bit,
in a smaller DP configuration.

In general, enabling x2apic with out enabling interrupt-remapping is not
recommended, as the platform will be using legacy interrupt format and the
CPU will be using extended ID mode.

thanks,
suresh
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