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[99.254.121.117]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-6c534329544sm23357966d6.22.2024.09.09.12.32.38 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 09 Sep 2024 12:32:39 -0700 (PDT) Date: Mon, 9 Sep 2024 15:32:37 -0400 From: Peter Xu To: "Maciej S. Szmigiero" Cc: Avihai Horon , Fabiano Rosas , Alex Williamson , =?utf-8?Q?C=C3=A9dric?= Le Goater , Eric Blake , Markus Armbruster , Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= , Joao Martins , qemu-devel@nongnu.org Subject: Re: [PATCH v2 06/17] migration: Add save_live_complete_precopy_{begin,end} handlers Message-ID: References: <9f9f0408-866d-4dc8-8c8e-80c3cfdd190a@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Received-SPF: pass client-ip=170.10.133.124; envelope-from=peterx@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -21 X-Spam_score: -2.2 X-Spam_bar: -- X-Spam_report: (-2.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.141, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=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 Mon, Sep 09, 2024 at 03:08:40PM -0400, Peter Xu wrote: > On Mon, Sep 09, 2024 at 08:32:45PM +0200, Maciej S. Szmigiero wrote: > > On 9.09.2024 19:59, Peter Xu wrote: > > > On Thu, Sep 05, 2024 at 04:45:48PM +0300, Avihai Horon wrote: > > > > > > > > On 27/08/2024 20:54, Maciej S. Szmigiero wrote: > > > > > External email: Use caution opening links or attachments > > > > > > > > > > > > > > > From: "Maciej S. Szmigiero" > > > > > > > > > > These SaveVMHandlers help device provide its own asynchronous > > > > > transmission of the remaining data at the end of a precopy phase. > > > > > > > > > > In this use case the save_live_complete_precopy_begin handler might > > > > > be used to mark the stream boundary before proceeding with asynchronous > > > > > transmission of the remaining data while the > > > > > save_live_complete_precopy_end handler might be used to mark the > > > > > stream boundary after performing the asynchronous transmission. > > > > > > > > > > Signed-off-by: Maciej S. Szmigiero > > > > > --- > > > > > include/migration/register.h | 36 ++++++++++++++++++++++++++++++++++++ > > > > > migration/savevm.c | 35 +++++++++++++++++++++++++++++++++++ > > > > > 2 files changed, 71 insertions(+) > > > > > > > > > > diff --git a/include/migration/register.h b/include/migration/register.h > > > > > index f60e797894e5..9de123252edf 100644 > > > > > --- a/include/migration/register.h > > > > > +++ b/include/migration/register.h > > > > > @@ -103,6 +103,42 @@ typedef struct SaveVMHandlers { > > > > > */ > > > > > int (*save_live_complete_precopy)(QEMUFile *f, void *opaque); > > > > > > > > > > + /** > > > > > + * @save_live_complete_precopy_begin > > > > > + * > > > > > + * Called at the end of a precopy phase, before all > > > > > + * @save_live_complete_precopy handlers and before launching > > > > > + * all @save_live_complete_precopy_thread threads. > > > > > + * The handler might, for example, mark the stream boundary before > > > > > + * proceeding with asynchronous transmission of the remaining data via > > > > > + * @save_live_complete_precopy_thread. > > > > > + * When postcopy is enabled, devices that support postcopy will skip this step. > > > > > + * > > > > > + * @f: QEMUFile where the handler can synchronously send data before returning > > > > > + * @idstr: this device section idstr > > > > > + * @instance_id: this device section instance_id > > > > > + * @opaque: data pointer passed to register_savevm_live() > > > > > + * > > > > > + * Returns zero to indicate success and negative for error > > > > > + */ > > > > > + int (*save_live_complete_precopy_begin)(QEMUFile *f, > > > > > + char *idstr, uint32_t instance_id, > > > > > + void *opaque); > > > > > + /** > > > > > + * @save_live_complete_precopy_end > > > > > + * > > > > > + * Called at the end of a precopy phase, after @save_live_complete_precopy > > > > > + * handlers and after all @save_live_complete_precopy_thread threads have > > > > > + * finished. When postcopy is enabled, devices that support postcopy will > > > > > + * skip this step. > > > > > + * > > > > > + * @f: QEMUFile where the handler can synchronously send data before returning > > > > > + * @opaque: data pointer passed to register_savevm_live() > > > > > + * > > > > > + * Returns zero to indicate success and negative for error > > > > > + */ > > > > > + int (*save_live_complete_precopy_end)(QEMUFile *f, void *opaque); > > > > > > > > Is this handler necessary now that migration core is responsible for the > > > > threads and joins them? I don't see VFIO implementing it later on. > > > > > > Right, I spot the same thing. > > > > > > This series added three hooks: begin, end, precopy_thread. > > > > > > What I think is it only needs one, which is precopy_async. My vague memory > > > was that was what we used to discuss too, so that when migration precopy > > > flushes the final round of iterable data, it does: > > > > > > (1) loop over all complete_precopy_async() and enqueue the tasks if > > > existed into the migration worker pool. Then, > > > > > > (2) loop over all complete_precopy() like before. > > > > > > Optionally, we can enforce one vmstate handler only provides either > > > complete_precopy_async() or complete_precopy(). In this case VFIO can > > > update the two hooks during setup() by detecting multifd && !mapped_ram && > > > nocomp. > > > > > > > The "_begin" hook is still necessary to mark the end of the device state > > sent via the main migration stream (during the phase VM is still running) > > since we can't start loading the multifd sent device state until all of > > that earlier data finishes loading first. > > Ah I remembered some more now, thanks. > > If vfio can send data during iterations this new hook will also not be > needed, right? > > I remember you mentioned you'd have a look and see the challenges there, is > there any conclusion yet on whether we can use multifd even during that? > > It's also a pity that we introduce this hook only because we want a > boundary between "iterable stage" and "final stage". IIUC if we have any > kind of message telling dest before hand that "we're going to the last > stage" then this hook can be avoided. Now it's at least inefficient > because we need to trigger begin() per-device, even if I think it's more of > a global request saying that "we need to load all main stream data first > before moving on". Or, we could add one MIG_CMD_SWITCHOVER under QEMU_VM_COMMAND, then send it at the beginning of the switchover phase. Then we can have a generic marker on destination to be the boundary of "iterations" v.s. "switchover". Then I think we can also drop the begin() here, just to avoid one such sync per-device (also in case if others may have such need, like vdpa, then vdpa doesn't need that flag too). Fundamentally, that makes the VFIO_MIG_FLAG_DEV_DATA_STATE_COMPLETE to be a migration flag.. But for sure the best is still if VFIO can enable multifd even during iterations. Then the boundary guard may not be needed. > > > > > We shouldn't send that boundary marker in .save_live_complete_precopy > > either since it would meant unnecessary waiting for other devices > > (not necessary VFIO ones) .save_live_complete_precopy bulk data. > > > > And VFIO SaveVMHandlers are shared for all VFIO devices (and const) so > > we can't really change them at runtime. > > In all cases, please consider dropping end() if it's never used; IMO it's > fine if there is only begin(), and we shouldn't keep hooks that are never > used. > > Thanks, > > -- > Peter Xu -- Peter Xu