[Qemu-devel] [PATCH v7 00/10] i8254, i8259 and running Microport UNIX (ca 1987)

[Matthew Ogilvie]

This series makes a series of mostly-unrelated fixes to allow
running an old Microport UNIX (ca 1987) guest under qemu.

Changes since version 6:
   * Patches 1 through 6 haven't changed, other than resolving
     a couple of simple conflicts.
   * Patch 7 "fixes" IRQ0 by just making it work like before,
     rather than fixing it properly.  This avoids possible risk
     to cross-version migration, etc.
   * Patches 8, 9, and 10 provide one possible gradual transition path
     to properly fix the 8254 model with relatively little risk to
     migration/etc.  The idea is that 8 and 9 could be applied
     immediately in preparation for a future fix, and then the
     actual fix (10) could be applied sometime in the future when
     migrating to or from pre-patch-9 versions is no longer a concern.
        I am not actually aware of ANY guest that actually needs
     an improved 8254 model, but this provides one way to improve
     it if desired.

----------------
Split up this series?

I'm not sure what the next steps are to get these into qemu, other
than waiting for 1.4 for at least the non-trivial parts?

Patches 1 through 3 could be considered independent trivial patches.
Would splitting them apart improve the changes of getting them into qemu?

Patch 4 isn't quite trivial, but it is well isolated (other than
small documentation conflicts against patch 3).  Should it be split
off?  It hasn't changed since version 3, but nobody has really
commented on it.

Patches 5 through 10 are interrelated, and should remain related in
a series.

----------------
Still needed:

  * Corresponding KVM patches.  The best approach may depend
    on what option is selected for qemu above.
     * Note that KVM uses a simplified model that doesn't try
       to emulate the trailing edge of the interrupt very well
       at all.  I'm not proposing to change this aspect of it.
     * A patch analogous to 7 should be easy.
     * Patches 8 through 10 are also fairly easy by themselves.
       But now we start having an explosion of combinations
       of versions of KVM and qemu and migration to/from, and it
       might be better to:
     * Or more involved fixes would involve new ioctl()'s and
       command line arguments to select old or fixed 8254 models
       dynamically.  See below.

----------------
Alternative options for improving the i8254 model and migration:

1. Don't fix 8254 at all.  Just apply through patch 7 or 8, and don't try
   to make any additional fixes.  I don't know of any guests that need
   improvements, so this could be a viable option.

2. Just fix it immediately, and don't worry about migration.  Squash
   the last few patches together.  A single missed periodic
   timer tick that only happens when migrating
   between versions of qemu is probably not a significant
   concern.  (Unless someone knows of an OS that actually runs
   the i8254 in single shot mode 4, where a missed interrupt
   could cause a hang or something?)

3. Use patches 8 and 9 now, and patch 10 sometime in the future.
   If it was just qemu, this would be attractive.  But when you
   also need to worry about a bunch of combinations of versions of
   qemu and KVM and migration, this is looking less attractive.

4. Support both old and fixed i8254 models, selectable at runtime
   with a command line option.  (Question: What should such an
   option look like?)  This may be the best way to actually
   change the 8254, but I'm not sure changes are even needed.
   It's certainly getting rather far afield from running Microport
   UNIX...

----------------

Matthew Ogilvie (10):
  fix some debug printf format strings
  vl: fix -hdachs/-hda argument order parsing issues
  qemu-options.hx: mention retrace= VGA option
  vga: add some optional CGA compatibility hacks
  i8259: fix so that dropping IRQ level always clears the interrupt
    request
  i8259: refactor pic_set_irq level logic
  i8254/i8259: workaround to make IRQ0 work like before
  i8254: add comments about fixing timings
  i8254: prepare for migration compatibility with future fixes
  FOR FUTURE: fix i8254/i8259 IRQ0 line logic

 hw/cirrus_vga.c   |  4 ++--
 hw/i8254.c        | 24 +++++++++++++++++++--
 hw/i8254_common.c | 18 ++++++----------
 hw/i8259.c        | 28 ++++++++++---------------
 hw/ide/cmd646.c   |  5 +++--
 hw/ide/via.c      |  5 +++--
 hw/pc.h           |  4 ++++
 hw/vga.c          | 37 ++++++++++++++++++++++++++-------
 qemu-options.hx   | 27 +++++++++++++++++++++++-
 vl.c              | 62 ++++++++++++++++++++++++++++++++++++-------------------
 10 files changed, 147 insertions(+), 67 deletions(-)

-- 
1.7.10.2.484.gcd07cc5