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

From: Peter Lieven
Subject: Re: [Qemu-devel] qemu-kvm-1.0.1 - unable to exit if vcpu is in infinite loop
Date: Thu, 28 Jun 2012 17:02:44 +0200
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20110921 Thunderbird/3.1.15

On 28.06.2012 15:25, Jan Kiszka wrote:
On 2012-06-28 15:05, Peter Lieven wrote:

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?
  - 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.


I failed reproducing so far.


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