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[99.254.121.117]) by smtp.gmail.com with ESMTPSA id y22-20020a05620a0e1600b0078ec71866f7sm1667171qkm.58.2024.04.19.08.31.27 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 19 Apr 2024 08:31:28 -0700 (PDT) Date: Fri, 19 Apr 2024 11:31:26 -0400 From: Peter Xu To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Cc: "Maciej S. Szmigiero" , Fabiano Rosas , Alex Williamson , =?utf-8?Q?C=C3=A9dric?= Le Goater , Eric Blake , Markus Armbruster , Avihai Horon , Joao Martins , qemu-devel@nongnu.org Subject: Re: [PATCH RFC 00/26] =?utf-8?Q?Multifd_?= =?utf-8?B?8J+UgA==?= device state transfer support with VFIO consumer Message-ID: References: <71ede5c8-857c-418b-9e37-b8d343ddfa06@maciej.szmigiero.name> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit 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: -43 X-Spam_score: -4.4 X-Spam_bar: ---- X-Spam_report: (-4.4 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-2.313, 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 Fri, Apr 19, 2024 at 11:07:21AM +0100, Daniel P. Berrangé wrote: > On Thu, Apr 18, 2024 at 04:02:49PM -0400, Peter Xu wrote: > > On Thu, Apr 18, 2024 at 08:14:15PM +0200, Maciej S. Szmigiero wrote: > > > I think one of the reasons for these results is that mixed (RAM + device > > > state) multifd channels participate in the RAM sync process > > > (MULTIFD_FLAG_SYNC) whereas device state dedicated channels don't. > > > > Firstly, I'm wondering whether we can have better names for these new > > hooks. Currently (only comment on the async* stuff): > > > > - complete_precopy_async > > - complete_precopy > > - complete_precopy_async_wait > > > > But perhaps better: > > > > - complete_precopy_begin > > - complete_precopy > > - complete_precopy_end > > > > ? > > > > As I don't see why the device must do something with async in such hook. > > To me it's more like you're splitting one process into multiple, then > > begin/end sounds more generic. > > > > Then, if with that in mind, IIUC we can already split ram_save_complete() > > into >1 phases too. For example, I would be curious whether the performance > > will go back to normal if we offloading multifd_send_sync_main() into the > > complete_precopy_end(), because we really only need one shot of that, and I > > am quite surprised it already greatly affects VFIO dumping its own things. > > > > I would even ask one step further as what Dan was asking: have you thought > > about dumping VFIO states via multifd even during iterations? Would that > > help even more than this series (which IIUC only helps during the blackout > > phase)? > > To dump during RAM iteration, the VFIO device will need to have > dirty tracking and iterate on its state, because the guest CPUs > will still be running potentially changing VFIO state. That seems > impractical in the general case. We already do such interations in vfio_save_iterate()? My understanding is the recent VFIO work is based on the fact that the VFIO device can track device state changes more or less (besides being able to save/load full states). E.g. I still remember in our QE tests some old devices report much more dirty pages than expected during the iterations when we were looking into such issue that a huge amount of dirty pages reported. But newer models seem to have fixed that and report much less. That issue was about GPU not NICs, though, and IIUC a major portion of such tracking used to be for GPU vRAMs. So maybe I was mixing up these, and maybe they work differently. Thanks, -- Peter Xu