Il 04/09/2012 11:51, Jan Kiszka ha scritto: >> > >> > I don't mean to say we shouldn't care about them, but there are likely >> > to be a lot more users doing backwards migration than users running >> > those guests, let alone migrating them (forwards or backwards). The >> > pragmatic choice is clear. > BTW, did anyone actually test backward migration recently? I thought to > remember I effectively broke it in 1.1 with some changes to the i8259 > (or was it the PIT?) vmstate, and no one really cared about this or my > first proposals to fix it.
Correct: commit ce967e2 (i8254: Rework & fix interaction with HPET in legacy mode, 2012-02-01) bumped the PIT version from 2 to 3. RTC changes will break it more in 1.3. Honestly, backwards migration only works on "enterprise" qemu-kvm because it is tested only there. And so far the only sample across major releases is that RHEL6->RHEL5 migration didn't work. Here the choice is between changing guest behavior by defaulting to 4/2, and always transmitting the subsection by defaulting to 0/0. The latter makes the subsection useless, so at that point we might as well bump the version number and board said flight to the Pacific. Paolo
