Re: migration: broken snapshot saves appear on s390 when small fields in migration stream removed

[Claudio Fontana]

On 7/15/20 1:10 PM, Claudio Fontana wrote:
> Hi Thomas,
> 
> On 7/14/20 4:35 PM, Thomas Huth wrote:
>> On 14/07/2020 16.29, Claudio Fontana wrote:
>>> Hello,
>>>
>>> I have some tiny progress in narrowing down this issue, possibly a qcow2 
>>> issue, still unclear,
>>> but involving Kevin Wolf and Max Reitz.
>>>
>>>
>>> The reproducer again:
>>>
>>>> --------------------------------------------cut-------------------------------------------
>>>> diff --git a/cpus.c b/cpus.c
>>>> index 41d1c5099f..443b88697a 100644
>>>> --- a/cpus.c
>>>> +++ b/cpus.c
>>>> @@ -643,7 +643,7 @@ static void qemu_account_warp_timer(void)
>>>>  
>>>>  static bool icount_state_needed(void *opaque)
>>>>  {
>>>> -    return use_icount;
>>>> +    return 0;
>>>>  }
>>>>  
>>>>  static bool warp_timer_state_needed(void *opaque)
>>>> --------------------------------------------cut-------------------------------------------
>>>
>>> This issue for now appears on s390 only:
>>>
>>> On s390 hardware, test 267 fails (both kvm and tcg) in the qcow2 backing 
>>> file part, with broken migration stream data in the s390-skeys vmsave (old 
>>> style).
>> [...]
>>> If someone has a good idea let me know - first attempts to reproduce on x86 
>>> failed, but maybe more work could lead to it.
>>
> 
> small update: in the GOOD case (enough padding added) a qcow_merge() is 
> triggered for the last write of 16202 bytes.
> In the BAD case (not enough padding added) a qcow_merge() is not triggered 
> for the last write of 16201 bytes.

I am talking about merge_cow() here, and it is more complicated. Will post a 
better explanation later on.

> 
> Note: manually flushing with qemu_fflush in s390-skeys vmsave also works 
> (maybe got lost in the noise).