Re: [Qemu-devel] fix clearing i8259 IRQ lines (Was: Should the i8259 devices remain no-user?)

[Matthew Ogilvie <mmogilvi_qemu@miniinfo.net>]

On Wed, Oct 16, 2013 at 06:23:11PM +0200, Paolo Bonzini wrote:
> Il 16/10/2013 18:21, BALATON Zoltan ha scritto:
> > A bit off topic but this reminded me of these patches:
> >
> > http://patchwork.ozlabs.org/patch/206753/
> > http://patchwork.ozlabs.org/patch/208252/
> >
> > which never got merged. Is there a chance that these fixes get merged
> > sometimes or is there an explanation why it won't be fixed? As far as I
> > remember the patches were reviewed and multiple versions were proposed
> > but at the end no decision was reached on which one to merge and it was
> > just left uncorrected.
>
> Right, thank you very much.  ISTR the unanswered question was what to do
> about migration, but I need to reread all the threads.
>
> Paolo

Essentially correct.

Although the 8259 (interrupts) model is clearly wrong with respect
to clearing an IRQ request line, only one ancient unimportant guest
(Microport UNIX ca. 1987) seems to care, and there are potentially
significant risks to more important guests if we try to fix it:

Risks: The 8254 (timers) model is wrong in various ways, some of
which are hidden by the incorrect 8259 model, and fixing it could
potentially break migration, depending on exact circumstances.
Also, it isn't clear if there are other device models depending
on the incorrect 8259 that would also need to be fixed.

Similar changes are needed in KVM for consistency, although some of
the 8254 modes are implemented in a more simplistic way (pulses
handled "as fast as possible" directly, instead of
1-millisecond-long pulses on real hardware).  Note that I was
never able to get my guest running successfully under KVM; I'm
not sure what the remaining problems were.

Also, the patch series included a few other things:
  - A couple of low priority fixes that can still be worked
    around without code changes, but could probably qualify
    as "trivial patches".
  - Some test cases to test for the 8259 problem.
  - Plus an optional VGA hack to make it work when
    my ancient guest tries to directly (no BIOS) configure it
    for CGA text mode.
I didn't get much feedback about these.

-----

If someone actually showed real interest in actually merging
these, including the selection of a migration compatibility
strategy they would actually be willing to merge (and above:
other devices, KVM, etc), I could look into updating
the patches to match.  But the "if" parts aren't looking
particularly likely.  This seems like a rather core-level
wide-implication change for a newbie to be messing
with.  (I've already spent noticably more time on qemu
patches than I had intended to spend total on playing with
this guest, although I may continue if I have a clearly
defined strategy.)

                - Matthew Ogilvie