[Qemu-devel] Re: [PATCH 4/4] KVM: Rework of guest debug state writing

[Jan Kiszka]

Jan Kiszka wrote:
> Marcelo Tosatti wrote:
>> On Thu, Feb 04, 2010 at 01:33:50AM +0100, Jan Kiszka wrote:
>>> Marcelo Tosatti wrote:
>>>> On Wed, Feb 03, 2010 at 10:29:45PM +0100, Jan Kiszka wrote:
>>>>> So far we synchronized any dirty VCPU state back into the kernel before
>>>>> updating the guest debug state. This was a tribute to a deficit in x86
>>>>> kernels before 2.6.33. But as this is an arch-dependent issue, it is
>>>>> better handle in the x86 part of KVM and remove the writeback point for
>>>>> generic code.
>>>> Jan,
>>>>
>>>> This patch breaks migration.
>>> Can you elaborate what you did? I can't reproduce, and I do not see any
>>> conceptual issue (given that guest debugging conflicts with migration
>>> anyway).
>> kvm-autotest fails (migration only, install is ok, both Linux and Win
>> guests). Not sure why, perhaps the unconditional KVM_SET_GUEST_DEBUG
>> corrupts state somehow? 
>>
>> Tested with io thread enabled.
> 
> That's this default-off thing, so... OK, confirmed, investigating.
> 

Heisenbug: It first also popped up (in form of a frozen migration
target) after removing this patch, but now it's totally unreproducible,
whatever patch I apply or revert from my series. Base is current master.
I tend to think there is a hidden issue of iothread vs. migration,
unrelated to this patch.

Jan

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