Re: [PATCH 1/1] poky: update qemu* to prefer 4.4 kernel

[Richard Purdie <richard.purdie@linuxfoundation.org>]

On Wed, 2016-03-09 at 13:53 -0500, Bruce Ashfield wrote:
> On 2016-03-01 8:41 PM, Paul Gortmaker wrote:
> > [Re: [poky] [PATCH 1/1] poky: update qemu* to prefer 4.4 kernel] On
> > 13/02/2016 (Sat 17:17) Richard Purdie wrote:
> > 
> > > I'm moving the discussion to OE-Core and pulling in some kernel
> > > people.
> > > I think I understand what is wrong and how to fix it but I could
> > > use
> > > someone who actually knows this code.
> > > 
> > > To summarise the story so far, on qemux86, X doesn't start and
> > > there is
> > > a backtrace in the logs:
> > > 
> > > x86/PAT: Xorg:705 map pfn expected mapping type uncached-minus
> > > for [mem 0xfd000000-0xfdffffff], got write-combining
> > 
> > So Bruce helped me set up a reproducer locally today since he'd
> > already
> > invested the time on that, and then I boiled that down to divorce
> > it
> > from the slower steps of build-deploy-boot to make the bisect
> > something
> > that mortal humans could tolerate.
> > 
> > Amusingly enough that led to:
> > 
> > commit 9cd25aac1f44f269de5ecea11f7d927f37f1d01c
> > Author: Borislav Petkov <bp@suse.de>
> > Date:   Thu Jun 4 18:55:10 2015 +0200
> > 
> >      x86/mm/pat: Emulate PAT when it is disabled
> > 
> > So while some of us were joking on IRC about the validity of
> > forcibly
> > disabling PAT (via cmdline or Kconfig) as a workaround, the one
> > line
> > shortlog above tells us that it wasn't so off the mark after all.
> > 
> > Bruce and I will decide what to do with this tomorrow, but since
> > Richard
> > spent so much time on it, I thought he'd like to know this in the
> > interim.  Good times.   :-/
> 
> As another follow up. The thread can be summarized as "It doesn't
> look like it should have worked before, and qemu's pat emulation
> may be the issue'.
> 
> The suggestion is to run with 'nopat', which is what Richard
> originally
> did.
> 
> So I'm going to prep a patch that drops the kernel patch, and leaves
> nopat enabled on the qemu command line. That should get us put back
> together in a semi-permanent way.

How sure are we this is a bug in QEMU's pat emulation? If that is the
case we should file a bug against qemu and try and fix it rather than
work around it...

Cheers,

Richard