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

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Date:	Sat, 04 Jul 2009 02:35:30 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Suresh Siddha <suresh.b.siddha@...el.com>,
	Gleb Natapov <gleb@...hat.com>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	Sheng Yang <sheng@...ux.intel.com>,
	"kvm\@vger.kernel.org" <kvm@...r.kernel.org>,
	"avi\@redhat.com" <avi@...hat.com>
Subject: Re: [PATCH v5] enable x2APIC without interrupt remapping under KVM

Ingo Molnar <mingo@...e.hu> writes:

> * 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.)

Please don't separate the x2apic code from the dmar code for this
reason.

Supporting hotplug cpus with ioapics is torture.

Eric
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