TeLeMan <gele...@gmail.com> wrote: > On Wed, Jul 28, 2010 at 18:43, Juan Quintela <quint...@redhat.com> wrote: >> TeLeMan <gele...@gmail.com> wrote: >>> If the new version adds the new subsection for some vmstate, the old >>> version will load the new version's vmstate unsuccessfully. So we have >>> to ignore the unrecognized subsections. >> >> No. That was the whole point of subsections. If one subsection is >> sent, target machine has to understand it. If it don't understand it, >> it fails. >> >> If subsection is not needed, it is the responsability of the source to >> not send it. >> >> This was one of the design requirements. Subsections are optional but >> it is the source which decides which ones to send/not to send. The >> target has to understand everything that it gets or fail the migration. >> > > If the target must understand everything, the vmstate's version will > be useless because the old version target maybe cannot load the new > version target's vmstate.
That is the whole point. See the ide example that I also sent. We have an old version of qemu, that don't understand pio ide fields. We get a new qemu version that has pio ide fields in the state. notice that migration in the middle of a pio operation is a very rare event. for instance, if you are using linux as a guest, pio is only used during boot, and not so many times (around 30000 calls during boot). Without subsections, you will be unable to migrate from new qemu to old qemu, because old qemu don't understand the pio fields. With subsections, if we are in the middle of a pio operation, we sent pio subsection and migration to old qemu will fail. But if we aren't in the middle of a pio operation, pio information will not be sent and migration will succeed. This is special important for the stable branch, where we are supposed to be backwards compatible, but sometimes we find a bug, and we really have to change the savevm format. This allows us that after creating the subsection, we can still migrate to the old version the majority of the time. Notice that this bugfixes are normally rare cases, because if it were the normal case, we would have already detected it before the release. If we sent the subsection, it means that target needs to understand it, or state will be broken (one way or another). I hope this hepls to understand how subsections are supposed to work. Thanks for the comments, Juan.
