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

From: ebiederm@xmission.com (Eric W. Biederman)
To: Gleb Natapov <gleb@redhat.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, 04 Jul 2009 07:33:39 -0700	[thread overview]
Message-ID: <m1r5wwa56k.fsf@fess.ebiederm.org> (raw)
In-Reply-To: <20090704095556.GB24641@redhat.com> (Gleb Natapov's message of "Sat\, 4 Jul 2009 12\:55\:56 +0300")

Gleb Natapov <gleb@redhat.com> writes:

> 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?

When I asked if cpu hotplug was a supported and more or
less common feature of kvm I was told it was.

Good cpu hotplug today means supporting interrupt remapping.
(The code you are disabling for kvm).

Therefore I don't see the point of supporting one without the other.
Especially if we don't have a case where on real hardware we need to
split the support.

Eric