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Date: Tue, 30 Jun 2009 12:26:23 +0300
From: Gleb Natapov <gleb@redhat.com>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
       Suresh Siddha <suresh.b.siddha@intel.com>,
       Sheng Yang <sheng@linux.intel.com>,
       "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
       "avi@redhat.com" <avi@redhat.com>
Subject: Re: [PATCH v3] enable x2APIC without interrupt remapping under KVM
Message-ID: <20090630092623.GI20289@redhat.com>
References: <20090629132926.GB20289@redhat.com> <m1ocs6axca.fsf@fess.ebiederm.org>
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On Tue, Jun 30, 2009 at 02:24:05AM -0700, Eric W. Biederman wrote:
> Gleb Natapov <gleb@redhat.com> writes:
> 
> > 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).
> 
> 
> How common is hotplug hardware in kvm?  In particular hotplug cpus?
> 
It works for Linux guests.

> To support that seriously you need interrupt remapping.
> 
Can you explain why?

--
			Gleb.