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[2003:cb:c707:5400:d8a3:8885:3275:4529]) by smtp.gmail.com with ESMTPSA id c5sm22809753wrq.102.2022.02.14.08.32.04 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 14 Feb 2022 08:32:05 -0800 (PST) Message-ID: <2743ef0f-c73c-5a55-067b-6068c23334f3@redhat.com> Date: Mon, 14 Feb 2022 17:32:03 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.4.0 To: Igor Mammedov References: <20220214133634.248d7de0@redhat.com> <20220214165515.226a1955@redhat.com> From: David Hildenbrand Organization: Red Hat Subject: Re: 9 TiB vm memory creation In-Reply-To: <20220214165515.226a1955@redhat.com> Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=david@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=170.10.129.124; envelope-from=david@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -28 X-Spam_score: -2.9 X-Spam_bar: -- X-Spam_report: (-2.9 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.083, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_LOW=-0.7, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Ani Sinha , Paolo Bonzini , QEMU Developers , kvm@vger.kernel.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" >>> >>> With KVM enabled it bails out with: >>> qemu-system-x86_64: kvm_set_user_memory_region: KVM_SET_USER_MEMORY_REGION failed, slot=1, start=0x100000000, size=0x8ff40000000: Invalid argument >>> >>> all of that on a host with 32G of RAM/no swap. >>> >>> >> >> #define KVM_MEM_MAX_NR_PAGES ((1UL << 31) - 1) >> >> ~8 TiB (7,999999) > > so essentially that's the our max for initial RAM > (ignoring initial RAM slots before 4Gb) > > Are you aware of any attempts to make it larger? Not really, I think for now only s390x had applicable machines where you'd have that much memory on a single NUMA node. > > But can we use extra pc-dimm devices for additional memory (with 8TiB limit) > as that will use another memslot? I remember that was the workaround for now for some extremely large VMs where you'd want a single NUMA node or a lot of memory for a single NUMA node. > > >> >> In QEMU, we have >> >> static hwaddr kvm_max_slot_size = ~0; >> >> And only s390x sets >> >> kvm_set_max_memslot_size(KVM_SLOT_MAX_BYTES); >> >> with >> >> #define KVM_SLOT_MAX_BYTES (4UL * TiB) > in QEMU default value is: > static hwaddr kvm_max_slot_size = ~0 > it is kernel side that's failing ... and kvm_set_max_memslot_size(KVM_SLOT_MAX_BYTES) works around the kernel limitation for s390x in user space. I feel like the right thing would be to look into increasing the limit in the kernel, and bail out if the kernel doesn't support it. Would require a new kernel for starting gigantic VMs with a single large memory backend, but then, it's a new use case. -- Thanks, David / dhildenb