linux-kernel - Re: [PATCH v5] enable x2APIC without interrupt remapping under KVM

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Date:	Fri, 3 Jul 2009 10:29:05 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	Gleb Natapov <gleb@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Sheng Yang <sheng@...ux.intel.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"avi@...hat.com" <avi@...hat.com>
Subject: Re: [PATCH v5] enable x2APIC without interrupt remapping under KVM


* Suresh Siddha <suresh.b.siddha@...el.com> wrote:

> On Wed, 2009-07-01 at 06:30 -0700, Gleb Natapov wrote:
> > KVM would like to provide x2APIC interface to a guest without emulating
> > interrupt remapping device. The reason KVM prefers guest to use x2APIC
> > is that x2APIC interface is better virtualizable and provides better
> > performance than mmio xAPIC interface:
> >     
> > - msr exits are faster than mmio (no page table walk, emulation)
> > - no need to read back ICR to look at the busy bit
> > - one 64 bit ICR write instead of two 32 bit writes
> > - shared code with the Hyper-V paravirt interface
> >     
> > Included patch changes x2APIC enabling logic to enable it even if IR
> > initialization failed, but kernel runs under KVM and no apic id is
> > greater than 255 (if there is one spec requires BIOS to move to x2apic
> > mode before starting an OS).
> > 
> > Signed-off-by: Gleb Natapov <gleb@...hat.com>
> 
> Acked-by: Suresh Siddha <suresh.b.siddha@...el.com>

Now, since this affects core x86 APIC code non-trivially so should 
submitted to and go via the x86 tree. (Can prepare a special branch 
with just this change if KVM tree wants/needs to pull it before 
v2.6.32.)

	Ingo
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