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

[Marc Zyngier <marc.zyngier@arm.com>]

On 24/02/15 14:36, Richard W.M. Jones wrote:
> 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?

Looks like it. Is your .config stashed somewhere? I'd like to give it a
go on my own setup...

> 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.

That'd be interesting indeed.

	M.
-- 
Jazz is not dead. It just smells funny...