From mboxrd@z Thu Jan 1 00:00:00 1970 From: Yang Zhang Subject: Re: Enable more than 255 VCPU support without irq remapping function in the guest Date: Wed, 27 Apr 2016 17:39:28 +0800 Message-ID: <572088D0.7040805@gmail.com> References: <571F93CA.40200@intel.com> <571F9487.5090009@siemens.com> <20160426164939.GA18900@potion> <57203B9D.6020402@gmail.com> <57204D28.4070706@siemens.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed 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: Jan Kiszka , =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= Return-path: Received: from mail-pa0-f65.google.com ([209.85.220.65]:33268 "EHLO mail-pa0-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753506AbcD0Jjk (ORCPT ); Wed, 27 Apr 2016 05:39:40 -0400 Received: by mail-pa0-f65.google.com with SMTP id vv3so4614038pab.0 for ; Wed, 27 Apr 2016 02:39:39 -0700 (PDT) In-Reply-To: <57204D28.4070706@siemens.com> Sender: kvm-owner@vger.kernel.org List-ID: 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 t= he >>>>> both >>>>> KVM/Xen. For some HPC cases, it needs many vcpus. The job require= s to >>>>> use X2APIC in the guest which supports 32-bit APIC id. Linux kern= el >>>>> requires irq remapping function during enabling X2APIC when max A= PIC 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~. IOAP= IC >>>>> 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 ena= ble >>>>> the >>>>> function, it seems a huge job which need to rework IO-APIC, local= APIC, >>>>> MSI parts and add virtual VTD support in the KVM. >>>>> >>>>> Other quick way to enable more than 256 VCPUs is to eliminate the >>>>> dependency between irq remapping and X2APIC in the guest linux ke= rnel. >>>>> 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 v= cpus >>>>> but 256 vcpus seem enough to balance irq requests in the guest. I= n the >>>>> most cases, there are fewer devices in the guest. >>>>> >>>>> I wonder whether it's feasible. There maybe some other side effec= ts I >>>>> didn't think of. Very appreciate for your comments. >>>> >>>> Radim is working on the KVM side already, Peter is currently drivi= ng the >>>> VT-d interrupt emulation topic in QEMU. It's in reach, I would say= =2E :) >>> >>> + Igor extends QEMU to support more than 255 in internal structures= 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 thi= nk >> firmware is able to send IPI to wakeup APs even without IR and OS is >> able to do it too. So basically, only KVM and Qemu's support is enou= gh. > > What are "internal devices" for you? And which OS do you know that wo= uld > handle such artificial setups without prio massive patching? Sorry, a typo. I mean the external devices of IOAPIC/MSI/MSIX. Doesn't=20 current Linux use x2apic without IR in VM? > > We do need VT-d IR emulation in order to present our guest a well > specified and support architecture for running > 255 CPUs. I mean in Tianyu's case, if he doesn't care about to deliver external=20 interrupt to CPU >255, IR is not required. --=20 best regards yang