Re: [Qemu-devel] problems with freeBSD

[Kevin O'Connor <kevin@koconnor.net>]

On Thu, Mar 07, 2013 at 09:43:04AM +0100, Aurelien Jarno wrote:
> On Wed, Mar 06, 2013 at 07:53:51PM -0500, Kevin O'Connor wrote:
> > That change is definitely just build related - I don't see how it
> > could impact the final SeaBIOS binary.  How did you conclude that this
> > commit is what fixes the issue?
> > 
> 
> I did a git bisect to find the commit fixing the issue. Then, as I was
> not believing the result, I tried the following sequence a dozen of
> times (for some unknown reasons the FreeBSD install CD doesn't exhibit
> the issue, so I used the Debian GNU/kFreeBSD installer):
[...]

Thanks for the detailed bug report.  Here's what I see going on:

- the SeaBIOS 4219149a commit does change the resulting binary ever so
  slightly - the src/virtio_ring.c code has a reference to __FILE__
  (the only code in SeaBIOS that does that), and due to slightly
  different build rules in this commit it evaluates to a slightly
  different string.

- the freebsd crash has nothing to do with 4219149a or
  src/virtio_ring.c - instead, random changes in the seabios binary
  layout can cause (or avoid) the crash.  You can see this in action
  by modifying seabios to have higher debug levels, commenting out
  code, adding dprintf statements, etc.

- the crash happens when freebsd attempts to emulate the bios code (!)
  in order to determine the keyboard typematic rate (!).  (See
  sys/dev/atkbdc/atkbd.c.) Since SeaBIOS doesn't support the typematic
  callback rate (int 0x16 ax=0x0306) this doesn't actually achieve
  anything in practice were the call to not crash.  However, a crash
  does (sometimes) result.

- the freebsd x86bios_get_pages() code is buggy (See
  sys/compat/x86bios/x86bios.c).  It attempts to check that its x86
  emulater (!) doesn't access a page it hasn't mapped.  However, it
  does not check for the case where a two byte access spans two pages.
  If the first page is mapped, but the second is not - splat.  The
  crash I've seen in QEMU had a two byte access to 0xffffff8000015fff
  with the fault at 0xffffff8000016000.

- I have not been able to determine why an attempt was made to access
  a non-mapped page.  My best guess is that the x86emu code (!) goes
  off the deep-end in all cases - just some cases lead it to the bug
  above and other cases lead it to a more friendly termination.
  (Recall that SeaBIOS doesn't support the typematic call anyway.)  It
  should be possible to track this down by adding debug statements to
  the freebsd code if anyone is familiar with the freebsd kernel
  compile-deploy-run cycle.

-Kevin