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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Mon, 30 Sep 2019 11:04:37 +0100 Received: from d06av24.portsmouth.uk.ibm.com (d06av24.portsmouth.uk.ibm.com [9.149.105.60]) by b06avi18878370.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x8UA4a1r31064472 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Mon, 30 Sep 2019 10:04:36 GMT Received: from d06av24.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id D78CE42042; Mon, 30 Sep 2019 10:04:36 +0000 (GMT) Received: from d06av24.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 877FA42047; Mon, 30 Sep 2019 10:04:36 +0000 (GMT) Received: from oc7455500831.ibm.com (unknown [9.152.224.146]) by d06av24.portsmouth.uk.ibm.com (Postfix) with ESMTP; Mon, 30 Sep 2019 10:04:36 +0000 (GMT) Subject: Re: [PATCH v7 4/4] s390: do not call memory_region_allocate_system_memory() multiple times To: pbonzini@redhat.com References: <20190924144751.24149-1-imammedo@redhat.com> <20190924144751.24149-5-imammedo@redhat.com> <20190925032700.GI28074@xz-x1> <20190925135105.6e5f249a@redhat.com> <20190925235235.GV28074@xz-x1> <20190927153320.2edc683c@redhat.com> <20190928012808.GA31218@xz-x1> <63e706b4-4a6a-3be5-6bb7-9c744d269d98@de.ibm.com> <20190930113329.71cab280@redhat.com> From: Christian Borntraeger Openpgp: preference=signencrypt Autocrypt: addr=borntraeger@de.ibm.com; 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Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <20190930113329.71cab280@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 x-cbid: 19093010-0008-0000-0000-0000031C5EF5 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19093010-0009-0000-0000-00004A3B047A Message-Id: <475f2019-4cc2-5d55-06de-99ea907c4d28@de.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:, , definitions=2019-09-30_06:, , signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=1 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1908290000 definitions=main-1909300109 X-detected-operating-system: by eggs.gnu.org: GNU/Linux 3.x [generic] [fuzzy] X-Received-From: 148.163.158.5 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: thuth@redhat.com, david@redhat.com, cohuck@redhat.com, qemu-devel@nongnu.org, Peter Xu , qemu-s390x@nongnu.org, Igor Mammedov Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 30.09.19 11:33, Igor Mammedov wrote: > On Mon, 30 Sep 2019 09:09:59 +0200 > Christian Borntraeger wrote: > >> On 28.09.19 03:28, Peter Xu wrote: >>> On Fri, Sep 27, 2019 at 03:33:20PM +0200, Igor Mammedov wrote: >>>> On Thu, 26 Sep 2019 07:52:35 +0800 >>>> Peter Xu wrote: >>>> >>>>> On Wed, Sep 25, 2019 at 01:51:05PM +0200, Igor Mammedov wrote: >>>>>> On Wed, 25 Sep 2019 11:27:00 +0800 >>>>>> Peter Xu wrote: >>>>>> >>>>>>> On Tue, Sep 24, 2019 at 10:47:51AM -0400, Igor Mammedov wrote: >>>>>>>> s390 was trying to solve limited KVM memslot size issue by abusing >>>>>>>> memory_region_allocate_system_memory(), which breaks API contract >>>>>>>> where the function might be called only once. >>>>>>>> >>>>>>>> Beside an invalid use of API, the approach also introduced migration >>>>>>>> issue, since RAM chunks for each KVM_SLOT_MAX_BYTES are transferred in >>>>>>>> migration stream as separate RAMBlocks. >>>>>>>> >>>>>>>> After discussion [1], it was agreed to break migration from older >>>>>>>> QEMU for guest with RAM >8Tb (as it was relatively new (since 2.12) >>>>>>>> and considered to be not actually used downstream). >>>>>>>> Migration should keep working for guests with less than 8TB and for >>>>>>>> more than 8TB with QEMU 4.2 and newer binary. >>>>>>>> In case user tries to migrate more than 8TB guest, between incompatible >>>>>>>> QEMU versions, migration should fail gracefully due to non-exiting >>>>>>>> RAMBlock ID or RAMBlock size mismatch. >>>>>>>> >>>>>>>> Taking in account above and that now KVM code is able to split too >>>>>>>> big MemorySection into several memslots, partially revert commit >>>>>>>> (bb223055b s390-ccw-virtio: allow for systems larger that 7.999TB) >>>>>>>> and use kvm_set_max_memslot_size() to set KVMSlot size to >>>>>>>> KVM_SLOT_MAX_BYTES. >>>>>>>> >>>>>>>> 1) [PATCH RFC v2 4/4] s390: do not call memory_region_allocate_system_memory() multiple times >>>>>>>> >>>>>>>> Signed-off-by: Igor Mammedov >>>>>>> >>>>>>> Acked-by: Peter Xu >>>>>>> >>>>>>> IMHO it would be good to at least mention bb223055b9 in the commit >>>>>>> message even if not with a "Fixed:" tag. May be amended during commit >>>>>>> if anyone prefers. >>>>>> >>>>>> /me confused, bb223055b9 is mentioned in commit message >>>>> >>>>> I'm sorry, I overlooked that. >>>>> >>>>>> >>>>>>> Also, this only applies the split limitation to s390. Would that be a >>>>>>> good thing to some other archs as well? >>>>>> >>>>>> Don't we have the similar bitmap size issue in KVM for other archs? >>>>> >>>>> Yes I thought we had. So I feel like it would be good to also allow >>>>> other archs to support >8TB mem as well. Thanks, >>>> Another question, Is there another archs with that much RAM that are >>>> available/used in real life (if not I'd wait for demand to arise first)? >>> >>> I don't know, so it was a pure question besides the series. Sorry if >>> that holds your series somehow, it was not my intention. >>> >>>> >>>> If we are to generalize it to other targets, then instead of using >>>> arbitrary memslot max size per target, we could just hardcode or get >>>> from KVM, max supported size of bitmap and use that to calculate >>>> kvm_max_slot_size depending on target page size. >>> >>> Right, I think if so hard code would be fine for now, and probably can >>> with a smallest one across all archs (should depend on the smallest >>> page size, I guess). >>> >>>> >>>> Then there wouldn't be need for having machine specific code >>>> to care about it and pick/set arbitrary values. >>>> >>>> Another aspect to think about if we are to enable it for >>>> other targets is memslot accounting. It doesn't affect s390 >>>> but other targets that support memory hotplug now assume 1:1 >>>> relation between memoryregion:memslot, which currently holds >>>> true but would need to amended in case split is enabled there. >>> >>> I didn't know this. So maybe it makes more sense to have s390 only >>> here. Thanks, >> >> OK. So shall I take the series as is via the s390 tree? > Yes, I'd appreciate it. Paolo, ok it I pick the first 3 patches as well? Can you ack?