Re: Can we force a KVM VCPU in Guest Mode to Exit to User Mode From User Mode ?

[Avi Kivity <avi@redhat.com>]

On 07/26/2012 04:39 PM, Mian M. Hamayun wrote:
>>
>> Can you share your reproducer?
> Actually its based on kvm-tool and I have integrated some code from
> qemu-kvm to add debug support to kvm-tool.
> I don't have a smaller example that could reproduce the same problem.

Then there's probably a bug there.

> 
> The thread 3 does actually receives the signal, but the order is not right.
> 
> As the BSP (Thread 2) starts, it locks the "qemu_global_mutex" and
> releases it only when it calls the "run_on_cpu" function and starts
> waiting on "qemu_work_cond".
> The AP (Thread 3) wakes-up due to the SIG_IPI signal from thread 2,
> acquires the lock on "qemu_global_mutex" and enters the guest mode.
> (This is the deadlock case)

Why would it enter guest mode?  Shouldn't it execute the function you
queued?

> 
> If we do not lock the "qemu_global_mutex" in each cpu thread at the
> beginning, and only lock it when we quit the guest mode, the problem
> seems to go away, as now we get the SIG_IPI when the Thread 3 is
> actually in the guest mode and it quits to user mode.
> But I am not sure if this is the right way to do it, as in qemu-kvm we
> _always_ start each cpu thread by locking the "qemu_global_mutex".

Do you call KVM_SET_SIGNAL_MASK to allow the signal?



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