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Mon, 29 Jun 2020 08:39:24 +0000 (UTC) Date: Mon, 29 Jun 2020 09:39:22 +0100 From: "Dr. David Alan Gilbert" To: Markus Armbruster Subject: Re: [PATCH v2 00/16] Crazy shit around -global (pardon my french) Message-ID: <20200629083922.GC2908@work-vm> References: <20200622094227.1271650-1-armbru@redhat.com> <87zh8r7nzd.fsf@dusky.pond.sub.org> <3ca757d9-39ff-677e-8062-729c3cb9db42@redhat.com> <874kqwr953.fsf@dusky.pond.sub.org> MIME-Version: 1.0 In-Reply-To: <874kqwr953.fsf@dusky.pond.sub.org> User-Agent: Mutt/1.14.3 (2020-06-14) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=dgilbert@redhat.com 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=207.211.31.81; envelope-from=dgilbert@redhat.com; helo=us-smtp-delivery-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/06/29 01:03:51 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: kwolf@redhat.com, berrange@redhat.com, ehabkost@redhat.com, qemu-block@nongnu.org, qemu-devel@nongnu.org, mreitz@redhat.com, pbonzini@redhat.com, John Snow Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" * Markus Armbruster (armbru@redhat.com) wrote: > Cc: David for insurance against me spewing nonsense about migration. > > John Snow writes: > > > On 6/25/20 12:45 AM, Markus Armbruster wrote: > >> John Snow writes: > >> > >>> On 6/22/20 5:42 AM, Markus Armbruster wrote: > >>>> There are three ways to configure backends: > >>>> > >>>> * -nic, -serial, -drive, ... (onboard devices) > >>>> > >>>> * Set the property with -device, or, if you feel masochistic, with > >>>> -set device (pluggable devices) > >>>> > >>>> * Set the property with -global (both) > >>>> > >>>> The trouble is -global is terrible. > >>>> > >>>> It gets applied in object_new(), which can't fail. We treat failure > >>>> to apply -global as fatal error, except when hot-plugging, where we > >>>> treat it as warning *boggle*. I'm not addressing that today. > >>>> > >>>> Some code falls apart when you use both -global and the other way. > >>>> > >>>> To make life more interesting, we gave -drive two roles: with > >>>> interface type other than none, it's for configuring onboard devices, > >>>> and with interface type none, it's for defining backends for use with > >>>> -device and such. Since we neglect to require interface type none for > >>>> the latter, you can use one -drive in both roles. This confuses the > >>>> code about as much as you, dear reader, probably are by now. > >>>> > >>>> Because this still isn't interesting enough, there's yet another way > >>>> to configure backends, just for floppies: set the floppy controller's > >>>> property. Goes back to the time when floppy wasn't a separate device, > >>>> and involves some Bad Magic. Now -global can interact with itself! > >>>> > >>>> Digging through all this took me an embarrassing amount of time. > >>>> Hair, too. > >>>> > >>>> My patches reject some the silliest uses outright, and deprecate some > >>>> not so silly ones that have replacements. > >>>> > >>>> Apply on top of my "[PATCH v2 00/58] qdev: Rework how we plug into the > >>>> parent bus". > >>>> > >>> > >>> Oof. Thank you for your work in fixing our darkest corners. I sincerely > >>> appreciate it. > >>> > >>> The qdev tree ordering problems don't cause any issues for migration, do > >>> they? > >> > >> This series should only change device configuration, not device state or > >> its encoding in the migration stream. > >> > >> I'm not sure what you mean by "qdev tree ordering problems". Ist it > >> commit e8c9e65816 'qom: Make "info qom-tree" show children sorted'? > >> > >>> (I see you already sent a PR, so whatever!) > >> > >> A question that might avoid a later migration debugging session is > >> *never* "whatever"! > >> > > > > I thought I had read that one of these patches changes the order in > > which devices get instantiated, which I thought might change their QOM > > paths. Which I thought *might* have some ramifications for migration, > > but wasn't sure. > > Device instantiation order changes should not break migration. They shouldn't; although I only narrowly stopped a new device from making a mistake that would have made it dependent. Of course you do have to explicitly state PCI/USB slot IDs otherwise the allocation of those is order dependent. > The order in which devices appear in the migration stream should not > matter. Order in the stream is a separate issue; we have ways to enforce that; for example you want the interrupt controller to arrive before a device that will raise an interrupt. Dave Dave > > If it's just showing the same path outputs *sorted*, then there's no > > problem. > > > > Likely misread. > > > > --js -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK