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(p200300cbc708420065d94d0dbb6114c8.dip0.t-ipconnect.de. [2003:cb:c708:4200:65d9:4d0d:bb61:14c8]) by smtp.gmail.com with ESMTPSA id n14-20020a05600c3b8e00b003b49bd61b19sm21404155wms.15.2023.01.10.02.18.34 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 10 Jan 2023 02:18:34 -0800 (PST) Message-ID: <460b6ea0-67a6-891f-f8fb-a5f23e9985c4@redhat.com> Date: Tue, 10 Jan 2023 11:18:34 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.6.0 Content-Language: en-US To: Peter Xu Cc: qemu-devel@nongnu.org, "Dr . David Alan Gilbert" , Juan Quintela , "Michael S . Tsirkin" , Michal Privoznik References: <20221222110215.130392-1-david@redhat.com> <20221222110215.130392-2-david@redhat.com> <482fadb5-7420-e07b-982d-5b0f3e8c42f8@redhat.com> From: David Hildenbrand Organization: Red Hat Subject: Re: [PATCH v3 1/6] migration: Allow immutable device state to be migrated early (i.e., before RAM) In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed 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: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, 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_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 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: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On 09.01.23 20:54, Peter Xu wrote: > On Mon, Jan 09, 2023 at 03:34:48PM +0100, David Hildenbrand wrote: >> On 05.01.23 18:15, Peter Xu wrote: >>> On Thu, Jan 05, 2023 at 09:35:54AM +0100, David Hildenbrand wrote: >>>> On 04.01.23 18:23, Peter Xu wrote: >>>>> On Thu, Dec 22, 2022 at 12:02:10PM +0100, David Hildenbrand wrote: >>>>>> Migrating device state before we start iterating is currently impossible. >>>>>> Introduce and use qemu_savevm_state_start_precopy(), and use >>>>>> a new special migration priority -- MIG_PRI_POST_SETUP -- to decide whether >>>>>> state will be saved in qemu_savevm_state_start_precopy() or in >>>>>> qemu_savevm_state_complete_precopy_*(). >>>>> >>>>> Can something like this be done in qemu_savevm_state_setup()? >>>> >>>> Hi Peter, >>> >>> Hi, David, >>> >>>> >>>> Do you mean >>>> >>>> (a) Moving qemu_savevm_state_start_precopy() effectively into >>>> qemu_savevm_state_setup() >>>> >>>> (b) Using se->ops->save_setup() >>> >>> I meant (b). >>> >>>> >>>> I first tried going via (b), but decided to go the current way of using a >>>> proper vmstate with properties (instead of e.g., filling the stream >>>> manually), which also made vmdesc handling possible (and significantly >>>> cleaner). >>>> >>>> Regarding (a), I decided to not move logic of >>>> qemu_savevm_state_start_precopy() into qemu_savevm_state_setup(), because it >>>> looked cleaner to save device state with the BQL held and for background >>>> snapshots, the VM has been stopped. To decouple device state saving from the >>>> setup path, just like we do it right now for all vmstates. >>> >>> Is BQL required or optional? IIUC it's at least still not taken in the >>> migration thread path, only in savevm path. >>> >>>> >>>> Having that said, for virtio-mem, it would still work because that state is >>>> immutable once migration starts, but it felt cleaner to separate the setup() >>>> phase from actual device state saving. >>> >>> I get the point. My major concerns are: >>> >>> (1) The new migration priority is changing the semantic of original, >>> making it over-complicated >>> >>> (2) The new precopy-start routine added one more step to the migration >>> framework, while it's somehow overlapping (if not to say, mostly the >>> same as..) save_setup(). >>> >>> For (1): the old priority was only deciding the order of save entries in >>> the global list, nothing more than that. Even if we want to have a >>> precopy-start phase, I'd suggest we use something else and keep the >>> migration priority simple. Otherwise we really need serious documentation >>> for MigrationPriority and if so I'd rather don't bother and not reuse the >>> priority field. >>> >>> For (2), if you see there're a bunch of save_setup() that already does >>> things like transferring static data besides the device states. Besides >>> the notorious ram_save_setup() there's also dirty_bitmap_save_setup() which >>> also sends a bitmap during save_setup() and some others. It looks clean to >>> me to do it in the same way as we used to. >>> >>> Reusing vmstate_save() and vmsd structures are useful too which I totally >>> agree. So.. can we just call vmstate_save_state() in the save_setup() of >>> the other new vmsd of virtio-mem? >> >> >> I went halfway that way, by moving stuff into qemu_savevm_state_setup() >> and avoiding using a new migration priority. Seems to work: > > The whole point of my suggestion is not moving things into > qemu_savevm_state_setup(), but avoid introducing more complexity to the > migration framework if unnecessary, so keep the generic framework as simple > as possible. IMHO, the current approach is actually quite simple and clean. But ... > >> >> I think we could go one step further and perform it from a save_setup() callback, >> however, I'm not convinced that this gets particularly cleaner (vmdesc handling >> eventually). > > What I wanted to suggest is exactly trying to avoid vmsd handling. To be > more explicit, I mean: besides vmstate_virtio_mem_device_early, virtio-mem > can register with another new SaveVMHandlers with both save_setup and > load_setup registered, then e.g. in its save_setup(), one simply calls: ... I can see if it can be made working that way and how the result looks. I know that we use vmstate_save_state() from virtio code, but I don't remember using it in save_setup() from QEMU_VM_SECTION_START and not QEMU_VM_SECTION_FULL. There is this interesting bit in register_savevm_live(), which sets "se->is_ram = 1". qemu_save_device_state() will not include the state. As it's used by XEN, I don't particularly care. > > vmstate_save_state(f, &vmstate_virtio_mem_device_early, virtio_mem_dev, > NULL); > > I'm not sure whether the JSONWriter* is required in this case, maybe not > yet to at least make it work. It was required when handling vmstates the current way to make analyze-migration.py not bail out (which is a good thing because one can actually inspect the migration content): $ ./scripts/analyze-migration.py -f STATEFILE { "ram (2)": { "section sizes": { "0000:00:03.0/mem0": "0x0000000f00000000", "pc.ram": "0x0000000100000000", "/rom@etc/acpi/tables": "0x0000000000020000", "pc.bios": "0x0000000000040000", "0000:00:02.0/e1000.rom": "0x0000000000040000", "pc.rom": "0x0000000000020000", "/rom@etc/table-loader": "0x0000000000001000", "/rom@etc/acpi/rsdp": "0x0000000000001000" } }, "0000:00:03.0/virtio-mem-device-early (51)": { "tmp": "00 00 00 01 40 00 00 00 00 00 00 0f 00 00 00 00 00 00 00 00 00 20 00 00 00 00 00 00", "size": "0x0000000000000000", "bitmap": "00 00 00 00 00 00 00 00 00 00 ... }, "timer (0)": { ... > > We'll need to impl the load part, but then IIUC we don't need to touch the > migration framework at all, and we keep all similar things (like other > devices I mentioned) to be inside save_setup(). > > Would that work? Let me play with it. -- Thanks, David / dhildenb