linux-kernel - Re: [PATCH v3] enable x2APIC without interrupt remapping under KVM

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Message-ID: <m1ocs6axca.fsf@fess.ebiederm.org>
Date:	Tue, 30 Jun 2009 02:24:05 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Gleb Natapov <gleb@...hat.com>
Cc:	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Sheng Yang <sheng@...ux.intel.com>,
	"kvm\@vger.kernel.org" <kvm@...r.kernel.org>,
	"avi\@redhat.com" <avi@...hat.com>
Subject: Re: [PATCH v3] enable x2APIC without interrupt remapping under KVM

Gleb Natapov <gleb@...hat.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?

To support that seriously you need interrupt remapping.

Eric


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