From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jan Kiszka Subject: Re: Enable more than 255 VCPU support without irq remapping function in the guest Date: Wed, 27 Apr 2016 11:45:56 +0200 Message-ID: <57208A54.40502@siemens.com> References: <571F93CA.40200@intel.com> <571F9487.5090009@siemens.com> <20160426164939.GA18900@potion> <57203B9D.6020402@gmail.com> <57204D28.4070706@siemens.com> <572088D0.7040805@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: "Lan, Tianyu" , pbonzini@redhat.com, kvm@vger.kernel.org, tglx@linutronix.de, gleb@redhat.com, mst@redhat.com, x86@kernel.org, Peter Xu , Igor Mammedov To: Yang Zhang , =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= Return-path: Received: from david.siemens.de ([192.35.17.14]:41730 "EHLO david.siemens.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752111AbcD0Jrt (ORCPT ); Wed, 27 Apr 2016 05:47:49 -0400 In-Reply-To: <572088D0.7040805@gmail.com> Sender: kvm-owner@vger.kernel.org List-ID: On 2016-04-27 11:39, Yang Zhang wrote: > On 2016/4/27 13:24, Jan Kiszka wrote: >> On 2016-04-27 06:10, Yang Zhang wrote: >>> On 2016/4/27 0:49, Radim Kr=C4=8Dm=C3=A1=C5=99 wrote: >>>> 2016-04-26 18:17+0200, Jan Kiszka: >>>>> On 2016-04-26 18:14, Lan, Tianyu wrote: >>>>>> Hi All: >>>>>> >>>>>> Recently I am working on extending max vcpu to more than 256 on = the >>>>>> both >>>>>> KVM/Xen. For some HPC cases, it needs many vcpus. The job requir= es to >>>>>> use X2APIC in the guest which supports 32-bit APIC id. Linux ker= nel >>>>>> requires irq remapping function during enabling X2APIC when max >>>>>> APIC id >>>>>> is more than 255(More detail please see try_to_enable_x2apic()). >>>> >>>> Our of curiosity, how many VCPUs are you aiming at? >>>> >>>>>> The irq remapping function helps to deliver irq to cpu 255~. IOA= PIC >>>>>> just >>>>>> supports 8-bit target APIC id field and only can deliver irq to >>>>>> cpu 0~255. >>>>>> >>>>>> So far both KVM/Xen doesn't enable irq remapping function. If en= able >>>>>> the >>>>>> function, it seems a huge job which need to rework IO-APIC, loca= l >>>>>> APIC, >>>>>> MSI parts and add virtual VTD support in the KVM. >>>>>> >>>>>> Other quick way to enable more than 256 VCPUs is to eliminate th= e >>>>>> dependency between irq remapping and X2APIC in the guest linux >>>>>> kernel. >>>>>> So far I can boot the guest after removing the dependency. >>>>>> The side effect I thought is that irq only can deliver to 0~255 = vcpus >>>>>> but 256 vcpus seem enough to balance irq requests in the guest. = In >>>>>> the >>>>>> most cases, there are fewer devices in the guest. >>>>>> >>>>>> I wonder whether it's feasible. There maybe some other side effe= cts I >>>>>> didn't think of. Very appreciate for your comments. >>>>> >>>>> Radim is working on the KVM side already, Peter is currently >>>>> driving the >>>>> VT-d interrupt emulation topic in QEMU. It's in reach, I would sa= y. :) >>>> >>>> + Igor extends QEMU to support more than 255 in internal structure= s and >>>> ACPI. What remains mostly untracked is Seabios/OVMF. >>> >>> If we don't want the interrupt from internal device delivers to CPU >>>> 255, do we still need the VT-d interrupt remapping emulation? I th= ink >>> firmware is able to send IPI to wakeup APs even without IR and OS i= s >>> able to do it too. So basically, only KVM and Qemu's support is eno= ugh. >> >> What are "internal devices" for you? And which OS do you know that w= ould >> handle such artificial setups without prio massive patching? >=20 > Sorry, a typo. I mean the external devices of IOAPIC/MSI/MSIX. Doesn'= t > current Linux use x2apic without IR in VM? If and only if there only need to be 254 CPUs to be addressed. >=20 >> >> We do need VT-d IR emulation in order to present our guest a well >> specified and support architecture for running > 255 CPUs. >=20 > I mean in Tianyu's case, if he doesn't care about to deliver external > interrupt to CPU >255, IR is not required. What matters is the guest OS. See my other reply on this why this doesn't work, even for Linux. Jan --=20 Siemens AG, Corporate Technology, CT RDA ITP SES-DE Corporate Competence Center Embedded Linux