linux-kernel - Re: [PATCH] x86: Make x2APIC support depend on interrupt remapping or guest support

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Message-ID: <20140706172805.GF4399@minantech.com>
Date:	Sun, 6 Jul 2014 20:28:05 +0300
From:	Gleb Natapov <gleb@...nel.org>
To:	Jan Kiszka <jan.kiszka@....de>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86 <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	kvm <kvm@...r.kernel.org>
Subject: Re: [PATCH] x86: Make x2APIC support depend on interrupt remapping
 or guest support

On Sun, Jul 06, 2014 at 05:49:09PM +0200, Jan Kiszka wrote:
> On 2014-07-06 17:41, Gleb Natapov wrote:
> > On Sun, Jul 06, 2014 at 05:24:27PM +0200, Jan Kiszka wrote:
> >> On 2014-07-06 17:12, Gleb Natapov wrote:
> >>> On Sat, Jul 05, 2014 at 09:47:54AM +0200, Jan Kiszka wrote:
> >>>> From: Jan Kiszka <jan.kiszka@...mens.com>
> >>>>
> >>>> We are able to use x2APIC mode in the absence of interrupt remapping on
> >>>> certain hypervisors. So it if fine to disable IRQ_REMAP without having
> >>>> to give up x2APIC support.
> >>> FWIW I did similar thing back when I added x2apic to KVM:
> >>> http://linux.derkeiler.com/Mailing-Lists/Kernel/2009-06/msg14579.html
> >>> But was advised against it.
> >>
> >> I don't get the point from that thread.
> > It is not architectural to use x2apic without irq remapping, so Suresh
> 
> Slightly off-topic for this patch: Is this dependency documented
> somewhere? I guess there are broken systems where it can fail, but none
> of the (Intel) boxes I tried so far had problems with x2APIC mode on
> (given APIC IDs < 255) and IRQ remapping disabled.
Not sure I saw that written, but I was told many times that this is so.

--
			Gleb.
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