From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:55837) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fC755-0005qe-Cl for qemu-devel@nongnu.org; Fri, 27 Apr 2018 13:22:48 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1fC750-0002tW-Fj for qemu-devel@nongnu.org; Fri, 27 Apr 2018 13:22:47 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:39938 helo=mx1.redhat.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1fC750-0002tA-AF for qemu-devel@nongnu.org; Fri, 27 Apr 2018 13:22:42 -0400 Date: Fri, 27 Apr 2018 18:22:31 +0100 From: "Dr. David Alan Gilbert" Message-ID: <20180427172230.GN2608@work-vm> References: <20180420175721.18479-1-dgilbert@redhat.com> <20180425095312.GA14253@9.122.211.20> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180425095312.GA14253@9.122.211.20> Subject: Re: [Qemu-devel] [PATCH] migration: update docs List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Balamuruhan S Cc: qemu-devel@nongnu.org, quintela@redhat.com * Balamuruhan S (bala24@linux.vnet.ibm.com) wrote: > On Fri, Apr 20, 2018 at 06:57:21PM +0100, Dr. David Alan Gilbert (git) wrote: > > +- Busses and devices should be able to explicitly specify addresses when > > + instantiated, and management tools should use those. For example, > > + when hot adding USB devices it's important to specify the ports > > + and addresses, since implicit ordering based on the command line order > > + may be different on the destination. This can result in the > > + device state being loaded into the wrong device. > > Would you like to add a note about taking care of migrating drc states incase > of hot adding devices, that could ensure hotunplug device safely after > migration ? That's something Power specific as I understand, but I don't know any details of it. What would you say as a general warning ? > > +When we migrate a device, we save/load the state as a series > > +of fields. Some times, due to bugs or new functionality, we need to > > +change the state to store more/different information. Changing the migration > > +state saved for a device can break migration comppatibility unless > > +care is taken to use the appropriate techniques. In general QEMU tries > > +to maintain forward migration compaitibility (i.e. migrating from > > +QEMU n->n+1) and there are users who benefit from backwards compatibility > > typo - %s/backwards/backward Interesting, I'd always said 'backwards' and there's only one place I said 'backward', but I guess 'backward' is more consistent with 'forward' so I'll remove all the 's's > > +Firmware > > +======== > > + > > +Migration migrates the copies of RAM and ROM, and thus when running > > +on the destination it includes the firmware from the source. Even after > > +resetting a VM, the old firmware is used. Only once QEMU has been restarted > > typo with 2 spaces I seem to have 3 there; I use 2 in most places; so made it consistent. Thanks, Dave > Only after QEMU has been restarted the new firmware will be used. > > -- Bala > > > +is the new firmware in use. > > + > > +- Changes in firmware size can cause changes in the required RAMBlock size > > + to hold the firmware and thus migration can fail. In practice it's best > > + to pad firmware images to convenient powers of 2 with plenty of space > > + for growth. > > + > > +- Care should be taken with device emulation code so that newer > > + emulation code can work with older firmware to allow forward migration. > > + > > +- Care should be taken with newer firmware so that backwards migration > > + to older systems with older device emulation code will work. > > + > > +In some cases it may be best to tie specific firmware versions to specific > > +versioned machine types to cut down on the combinations that will need > > +support. This is also useful when newer versions of firmware outgrow > > +the padding. > > + > > -- > > 2.17.0 > > > > > -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK