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

From: Gleb Natapov <gleb@redhat.com>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Ingo Molnar <mingo@elte.hu>,
	Suresh Siddha <suresh.b.siddha@intel.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Sheng Yang <sheng@linux.intel.com>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	"avi@redhat.com" <avi@redhat.com>
Subject: Re: [PATCH v5] enable x2APIC without interrupt remapping under KVM
Date: Sat, 4 Jul 2009 12:55:56 +0300	[thread overview]
Message-ID: <20090704095556.GB24641@redhat.com> (raw)
In-Reply-To: <m1r5wwbxjx.fsf@fess.ebiederm.org>

On Sat, Jul 04, 2009 at 02:35:30AM -0700, Eric W. Biederman wrote:
> Ingo Molnar <mingo@elte.hu> writes:
> 
> > * Suresh Siddha <suresh.b.siddha@intel.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@redhat.com>
> >> 
> >> Acked-by: Suresh Siddha <suresh.b.siddha@intel.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.
> 
What is the connection between this patch and cpu hotplug?

--
			Gleb.