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

[Avi Kivity <avi@redhat.com>]

On 09/04/2012 01:06 PM, Paolo Bonzini wrote:
> 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.  

Gah, this is 1.1, so there's no way to fix it.

> 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.

I expect that we will want 7->6, though we may have to settle for minor
releases only.

> 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.

So we do the 4/2 thing.  I expect if we go they'll tell us they don't do
backwards flights.

-- 
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