Re: kvm [2087]: load/store instruction decoding not implemented

["Richard W.M. Jones" <rjones@redhat.com>]

On Tue, Feb 24, 2015 at 02:10:28PM +0000, Marc Zyngier wrote:
> On 24/02/15 13:45, Richard W.M. Jones wrote:
> > On Tue, Feb 24, 2015 at 01:12:49PM +0000, Marc Zyngier wrote:
> >> Here's my theory: userspace is accessing something it should never
> >> access (outside of RAM, basically), and doing so via a kernel interface.
> >>
> >> Is this process accessing /dev/mem by any chance? dmidecode anyone?
> > 
> > Not as far as I know.  The userspace process is inserting modules.
> > 
> > Here is the userspace function which is most likely to be running:
> > 
> > https://github.com/libguestfs/supermin/blob/master/src/init.c#L292
> 
> Hmmm. That seems quite inoffensive indeed...
> 
> > Unfortunately because of lack of a full stack trace, I can't be sure
> > exactly what system call is failing, but I'll probably add more debug
> > to the userspace program later.
> > 
> > BTW this worked fine in 3.19.  It's started failing in 3.20/4.0.  It
> > also works fine on x86.
> 
> Any chance you could find out whether that's a host or guest regression?

Here is a summary of the test combinations that I have run:

  guest kernel         host kernel         result
--------------------------------------------------------
  3.19.0-0.rc7         3.19.0-0.rc7        no bug seen

  3.20.0-0.rc0         3.19.0-0.rc7        bug seen

  3.19.0-0.rc7         4.0.0-0.rc1         no bug seen

  4.0.0-0.rc1          4.0.0-0.rc1         bug seen

So a guest regression, I think?

It's also possible the bug existed in old kernels but was masked
somehow, eg. different memory layout.

I can probably bisect this given time, but I'm going to try putting
some debug into the userspace process to find out which system call
fails first.

Rich.

-- 
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