linux-kernel - Re: [PATCH] x86: enable x2apic early at the first point

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Date:	Fri, 20 Feb 2009 13:59:19 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Gleb Natapov <gleb@...hat.com>
Cc:	Suresh Siddha <suresh.b.siddha@...el.com>,
	Yinghai Lu <yinghai@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: enable x2apic early at the first point


* Gleb Natapov <gleb@...hat.com> wrote:

> On Fri, Feb 20, 2009 at 12:06:51PM +0100, Ingo Molnar wrote:
> > > > > If you'll revert my patch it will not be possible to use 
> > > > > x2apic in KVM (at least without KVM implementing interrupt 
> > > > > remapping which is unneeded otherwise) and x2apic interface is 
> > > > > much better for vitalization. Instead of reverting the patch 
> > > > > it will be better to add check if x2apic can be used without 
> > > > > intr-remmaping (all CPUs belong to cluster 0) or allow 
> > > > > enabling of x2apic without IR if running as a guest.
> > > > 
> > > > yep, that would be required - because your patch can break real 
> > > > systems right now. Mind sending me a fix for it?
> > > > 
> > > > I've applied Yinghai's fix as well, so please base it on latest 
> > > > tip:master.
> > > > 
> > > OK. Will send next week.
> > 
> > ok, that's too long of a breakage window - i'll revert it then, 
> > please send a new version once you have it.
> > 
>
> What approach you actually prefer? Enable x2apic without 
> intr-remapping only when running in KVM, or even if running on 
> real HW but all CPUs are in cluster 0? If former then below is 
> updated patch (tested only in KVM) if later then it's 
> definitely for next week :)

The latter would be the ideal solution - i.e. next week :)

x2apic is a rare feature, and the more exposure we give it the 
more tested it becomes. It's also obviously good for general 
code structure if core CPU features are separated from 
chipset/iommu features.

Plus, x2apic accesses could actually be faster than UC accesses 
to the lapic, so whenever we can we should enter x2apic mode - 
even if intr-remap is not turned on (because not needed).

	Ingo
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