Re: [Qemu-devel] [PATCH v4 5/5] i8259: fix dynamically masking slave IRQs with IMR register

[Paolo Bonzini]

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