From mboxrd@z Thu Jan 1 00:00:00 1970 From: Thomas Huth Subject: Re: [Qemu-ppc] KVM memory slots limit on powerpc Date: Tue, 8 Sep 2015 11:22:09 +0200 Message-ID: <55EEA8C1.4050503@redhat.com> References: <55E965FC.8080909@redhat.com> <55E96B87.8020900@de.ibm.com> <55E96CB9.4070100@suse.de> <20150907163131.3cb11a7b@nial.brq.redhat.com> <55EE7AB5.1000407@redhat.com> <55EE8A0B.302@de.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Cc: kvm-ppc@vger.kernel.org, qemu-ppc@nongnu.org, kvm@vger.kernel.org, David Gibson , Alex Williamson , Bharata B Rao To: Christian Borntraeger , Igor Mammedov , Alexander Graf Return-path: Received: from mx1.redhat.com ([209.132.183.28]:44309 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754243AbbIHJWO (ORCPT ); Tue, 8 Sep 2015 05:22:14 -0400 In-Reply-To: <55EE8A0B.302@de.ibm.com> Sender: kvm-owner@vger.kernel.org List-ID: On 08/09/15 09:11, Christian Borntraeger wrote: > Am 08.09.2015 um 08:05 schrieb Thomas Huth: >> On 07/09/15 16:31, Igor Mammedov wrote: >>> On Fri, 4 Sep 2015 12:04:41 +0200 >>> Alexander Graf wrote: >>>> On 04.09.15 11:59, Christian Borntraeger wrote: >>>>> Am 04.09.2015 um 11:35 schrieb Thomas Huth: >>>>>> >>>>>> now that we get memory hotplugging for the spapr machine on qemu-ppc, >>>>>> too, it seems like we easily can hit the amount of KVM-internal memory >>>>>> slots now ("#define KVM_USER_MEM_SLOTS 32" in >>>>>> arch/powerpc/include/asm/kvm_host.h). For example, start >>>>>> qemu-system-ppc64 with a couple of "-device secondary-vga" and "-m >>>>>> 4G,slots=32,maxmem=40G" and then try to hot-plug all 32 DIMMs ... and >>>>>> you'll see that it aborts way earlier already. >>>>>> >>>>>> The x86 code already increased the amount of KVM_USER_MEM_SLOTS to 509 >>>>>> already (+3 internal slots = 512) ... maybe we should now increase the >>>>>> amount of slots on powerpc, too? Since we don't use internal slots on >>>>>> POWER, would 512 be a good value? Or would less be sufficient, too? >>>>> >>>>> When you are at it, the s390 value should also be increased I guess. >>>> >>>> That constant defines the array size for the memslot array in struct kvm >>>> which in turn again gets allocated by kzalloc, so it's pinned kernel >>>> memory that is physically contiguous. Doing big allocations can turn >>>> into problems during runtime. >>>> >>>> So maybe there is another way? Can we extend the memslot array size >>>> dynamically somehow? Allocate it separately? How much memory does the >>>> memslot array use up with 512 entries? >>> >>> KVM switched memslots allocation to kvm_kvzalloc(), so it would fallback to vmalloc >>> commit 744961341d472db6272ed9b42319a90f5a2aa7c4 >>> kvm: avoid page allocation failure in kvm_set_memory_region() >> >> Good hint, thanks for pointing that out! ... so increasing the array >> size should not cause too much trouble :-) > > Changing the allocation of the memslots to a growing structure seems like a > good idea nevertheless. Any chance to do this as well when you are at it? I'd like to finish some other stuff first, so if you want to have a try, feel free to do so ... if not, I'll have a closer look at this later. Thomas