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Szmigiero" Cc: Alex Williamson , Peter Xu , =?utf-8?Q?C=C3=A9dric?= Le Goater , Eric Blake , Markus Armbruster , =?utf-8?Q?Dan?= =?utf-8?Q?iel_P_=2E_Berrang=C3=A9?= , Avihai Horon , Joao Martins , qemu-devel@nongnu.org Subject: Re: [PATCH v2 13/17] migration/multifd: Add migration_has_device_state_support() In-Reply-To: <4aaad112-5fdf-4172-bfb4-65dd7880202d@maciej.szmigiero.name> References: <8407eb455dfc1dea3cabf065f90833fab337eb98.1724701542.git.maciej.szmigiero@oracle.com> <8734mlon65.fsf@suse.de> <4aaad112-5fdf-4172-bfb4-65dd7880202d@maciej.szmigiero.name> Date: Tue, 03 Sep 2024 12:09:23 -0300 Message-ID: <87a5gon598.fsf@suse.de> MIME-Version: 1.0 Content-Type: text/plain X-Rspamd-Queue-Id: 118081F397 X-Rspamd-Action: no action X-Spamd-Result: default: False [-6.51 / 50.00]; BAYES_HAM(-3.00)[99.99%]; DWL_DNSWL_MED(-2.00)[suse.de:dkim]; NEURAL_HAM_LONG(-1.00)[-1.000]; R_DKIM_ALLOW(-0.20)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; MX_GOOD(-0.01)[]; FROM_HAS_DN(0.00)[]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; RBL_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:104:10:150:64:97:from]; ARC_NA(0.00)[]; FUZZY_BLOCKED(0.00)[rspamd.com]; MIME_TRACE(0.00)[0:+]; SPAMHAUS_XBL(0.00)[2a07:de40:b281:104:10:150:64:97:from]; TO_MATCH_ENVRCPT_ALL(0.00)[]; RCVD_TLS_ALL(0.00)[]; DKIM_TRACE(0.00)[suse.de:+]; MID_RHS_MATCH_FROM(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; FROM_EQ_ENVFROM(0.00)[]; TO_DN_SOME(0.00)[]; RCPT_COUNT_SEVEN(0.00)[10]; RCVD_VIA_SMTP_AUTH(0.00)[]; RECEIVED_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:106:10:150:64:167:received]; MISSING_XM_UA(0.00)[]; DBL_BLOCKED_OPENRESOLVER(0.00)[imap1.dmz-prg2.suse.org:rdns, imap1.dmz-prg2.suse.org:helo, suse.de:dkim, suse.de:mid, suse.de:email] X-Rspamd-Server: rspamd1.dmz-prg2.suse.org Received-SPF: pass client-ip=2a07:de40:b251:101:10:150:64:2; envelope-from=farosas@suse.de; helo=smtp-out2.suse.de 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, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, 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: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org "Maciej S. Szmigiero" writes: > On 30.08.2024 20:55, Fabiano Rosas wrote: >> "Maciej S. Szmigiero" writes: >> >>> From: "Maciej S. Szmigiero" >>> >>> Since device state transfer via multifd channels requires multifd >>> channels with packets and is currently not compatible with multifd >>> compression add an appropriate query function so device can learn >>> whether it can actually make use of it. >>> >>> Signed-off-by: Maciej S. Szmigiero >> >> Reviewed-by: Fabiano Rosas >> >> Out of curiosity, what do you see as a blocker for migrating to a file? >> >> We would just need to figure out a mapping from file offset some unit of >> data to be able to write in parallel like with ram (of which the page >> offset is mapped to the file offset). > > I'm not sure whether there's a point in that since VFIO devices > just provide a raw device state stream - there's no way to know > that some buffer is no longer needed because it consisted of > dirty data that was completely overwritten by a later buffer. > > Also, the device type that the code was developed against - a (smart) > NIC - has so large device state because (more or less) it keeps a lot > of data about network connections passing / made through it. > > It doesn't really make sense to make snapshot of such device for later > reload since these connections will be long dropped by their remote > peers by this point. > > Such snapshotting might make more sense with GPU VFIO devices though. > > If such file migration support is desired at some later point then for > sure the whole code would need to be carefully re-checked for implicit > assumptions. Thanks, let's keep those arguments in mind, maybe we put them in a doc somewhere so we remember this in the future. > > Thanks, > Maciej