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Mon, 15 Jun 2020 14:45:28 +0000 (UTC) Received: by sirius.home.kraxel.org (Postfix, from userid 1000) id 1BFFA17449; Mon, 15 Jun 2020 16:45:27 +0200 (CEST) Date: Mon, 15 Jun 2020 16:45:27 +0200 From: Gerd Hoffmann To: Laszlo Ersek Subject: Re: [PATCH v7 2/5] softmmu/vl: Let -fw_cfg option take a 'gen_id' argument Message-ID: <20200615144527.w2637fq5jzpk7jtt@sirius.home.kraxel.org> References: <20200528173141.17495-1-philmd@redhat.com> <20200528173141.17495-3-philmd@redhat.com> <20200609155050.GT2880@minyard.net> <6e4b9540-7d5b-0951-2870-bada03e29d83@redhat.com> MIME-Version: 1.0 In-Reply-To: <6e4b9540-7d5b-0951-2870-bada03e29d83@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Received-SPF: pass client-ip=207.211.31.120; envelope-from=kraxel@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/06/15 01:17:36 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -30 X-Spam_score: -3.1 X-Spam_bar: --- X-Spam_report: (-3.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1, 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_H3=-0.01, RCVD_IN_MSPIKE_WL=-0.01, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=_AUTOLEARN X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Paolo Bonzini , Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= , qemu-devel@nongnu.org, minyard@acm.org, Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Hi, > > I can explain the rationale for that change, but I'm not sure of the > > answer to your question. That changes makes sure that the fw_cfg data > > remains exactly the same even on newer versions of qemu if the machine > > is set the same. This way you can do migrations to newer qemu versions > > and anything using fw_cfg won't get confused because the data changes. > > > > The reason that change was so complex was preserving the order for > > migrating from older versions. > > > > This is only about migration. I'm not sure what gen_id is, but if it's > > migrated, it better be future proof. > > Whenever introducing a new fw_cfg file (*any* new named file), how do we > decide whether we need fw_cfg_set_order_override()? The whole point of the sorting is to make sure the fw_cfg directory listing entry (FW_CFG_FILE_DIR) is stable and doesn't change underneath the guest when it gets live-migrated. That sorting was added in qemu 2.6, to make sure things don't chance by accident in case the initialization order changes. Now you've got a problem when you migrate from qemu 2.5 (+older) to qemu 2.6 (+newer), because 2.5 has the entries in initialization order and 2.6 has the entries in alphabetical order. To deal with that machine types for 2.5 & older keep the old sort order. This is the reason why legacy_fw_cfg_order exists. For new features and files you can completely ignore the whole legacy sorting mess. cross-version live migration works only for features supported by both qemu versions, therefore the legacy sorting is only relevant for features & files already supported by qemu 2.5. HTH, Gerd