Re: [Qemu-devel] qemu-kvm-1.0.1 - unable to exit if vcpu is in infinite loop

[Peter Lieven]

On 28.06.2012 15:25, Jan Kiszka wrote:
> On 2012-06-28 15:05, Peter Lieven wrote:
> > Hi,
> >
> > i debugged my initial problem further and found out that the problem
> > happens to be that
> > the main thread is stuck in pause_all_vcpus() on reset or quit commands
> > in the monitor
> > if one cpu is stuck in the do-while loop kvm_cpu_exec. If I modify the
> > condition from while (ret == 0)
> > to while ((ret == 0)&&  !env->stop); it works, but is this the right fix?
> > "Quit" command seems to work, but on "Reset" the VM enterns pause state.
> Before entering the wait loop in pause_all_vcpus, there are kicks sent
> to all vcpus. Now we need to find out why some of those kicks apparently
> don't reach the destination.
can you explain shot what exactly these kicks do? does these kicks lead
to leaving the kernel mode and returning to userspace?
> Again:
>   - on which host kernels does this occur, and which change may have
>     changed it?
I do not see it in 3.0.0 and have also not seen it in 2.6.38. both
the mainline 64-bit ubuntu-server kernels (for natty / oneiric 
respectively).
If I compile a more recent kvm-kmod 3.3 or 3.4 on these machines,
it is no longer working.
>   - with which qemu-kvm version is it reproducible, and which commit
>     introduced or fixed it?
qemu-kvm-1.0.1 from sourceforge. to get into the scenario it
is not sufficient to boot from an empty harddisk. to reproduce
i have use a live cd like ubuntu-server 12.04 and choose to
boot from the first harddisk. i think the isolinux loader does
not check for a valid bootsector and just executes what is found
in sector 0. this leads to the mmio reads i posted and 100%
cpu load (most spent in kernel). at that time the monitor/qmp
is still responsible. if i sent a command that pauses all vcpus,
the first cpu is looping in kvm_cpu_exec and the main thread
is waiting. at that time the monitor stops responding.
i have also seen this issue on very old windows 2000 servers
where the system fails to power off and is just halted. maybe
this is also a busy loop.

i will try to bisect this asap and let you know, maybe the above
info helps you already to reproduce.

thanks,
peter

> I failed reproducing so far.
>
> Jan