Re: [Qemu-devel] [PATCH RFC 1/1] vmstate: draft fix for failed iotests case 68 and 91

[Halil Pasic]

On 03/07/2017 11:05 AM, Kevin Wolf wrote:
> Am 07.03.2017 um 10:54 hat Halil Pasic geschrieben:
>>
>>
>> On 03/07/2017 10:29 AM, Kevin Wolf wrote:
>>> Am 07.03.2017 um 03:53 hat QingFeng Hao geschrieben:
>>>> I am not very clear about the logic in vmstate.c, but from its context in
>>>> vmstate_save_state, it seems size should not be 0, otherwise the followed
>>>> for loop will keep working on the same element. So I just add a simple
>>>> check to pass that case, not sure if it's right but it can pass iotest
>>>> case 68 and 91 now.
>>>>
>>>> The iotest's failed output is:
>>>> 068 1s ... - output mismatch (see 068.out.bad)
>>>> --- 
>>>> /home/haoqf/KVMonz/gitcheck/work/qemu-master/tree/qemu/tests/qemu-iotests/068.out
>>>>    2017-03-06 05:52:24.817328899 +0100
>>>> +++ 068.out.bad 2017-03-07 03:28:44.426714519 +0100
>>>> @@ -3,9 +3,13 @@
>>>>  === Saving and reloading a VM state to/from a qcow2 image ===
>>>>
>>>>  Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=131072
>>>> +qemu-system-s390x: migration/vmstate.c:336: vmstate_save_state: Assertion 
>>>> `first_elem || !n_elems' failed.
>>>> +./common.config: line 109: 52497 Aborted                 ( if [ -n 
>>>> "${QEMU_NEED_PID}" ]; then
>>>> +    echo $BASHPID > "${QEMU_TEST_DIR}/qemu-${_QEMU_HANDLE}.pid";
>>>> +fi; exec "$QEMU_PROG" $QEMU_OPTIONS "$@" )
>>>>  QEMU X.Y.Z monitor - type 'help' for more information
>>>>  (qemu) savevm 0
>>>> -(qemu) quit
>>>> +qemu-system-s390x: Device 'virtio0' does not have the requested snapshot 
>>>> '0'
>>>>  QEMU X.Y.Z monitor - type 'help' for more information
>>>>  (qemu) quit
>>>>  *** done
>>>>
>>>> 091 1s ... [failed, exit status 1] - output mismatch (see 091.out.bad)
>>>>     --- tests/qemu-iotests/091.out 2016-08-30 12:35:04.207683276 +0200
>>>>     +++ 091.out.bad        2017-03-06 13:08:03.717135426 +0100
>>>>     @@ -11,18 +11,23 @@
>>>>      
>>>>      vm1: qemu-io disk write complete
>>>>      vm1: live migration started
>>>>     -vm1: live migration completed
>>>>     -
>>>>     -=== VM 2: Post-migration, write to disk, verify running ===
>>>>     -
>>>>     -vm2: qemu-io disk write complete
>>>>     -vm2: qemu process running successfully
>>>>     -vm2: flush io, and quit
>>>>     -Check image pattern
>>>>     -read 4194304/4194304 bytes at offset 0
>>>>     -4 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
>>>>     -Running 'qemu-img check -r all $TEST_IMG'
>>>>     -No errors were found on the image.
>>>>     -80/16384 = 0.49% allocated, 0.00% fragmented, 0.00% compressed 
>>>> clusters
>>>>     -Image end offset: 5570560
>>>>     -*** done
>>>>     +./common.qemu: line 110: write error: Broken pipe
>>>>     +./common.qemu: line 110: write error: Broken pipe
>>>>     +./common.qemu: line 110: write error: Broken pipe
>>>>     +./common.qemu: line 110: write error: Broken pipe
>>>>     +./common.qemu: line 110: write error: Broken pipe
>>>>     +./common.qemu: line 110: write error: Broken pipe
>>>>     +./common.qemu: line 110: write error: Broken pipe
>>>>     +./common.qemu: line 110: write error: Broken pipe
>>>>     +./common.qemu: line 110: write error: Broken pipe
>>>>     +./common.qemu: line 110: write error: Broken pipe
>>>>     +./common.qemu: line 110: write error: Broken pipe
>>>>     +./common.qemu: line 110: write error: Broken pipe
>>>>     +./common.qemu: line 110: write error: Broken pipe
>>>>     +./common.qemu: line 110: write error: Broken pipe
>>>>     +./common.qemu: line 110: write error: Broken pipe
>>>>     +./common.qemu: line 110: write error: Broken pipe
>>>>     +./common.qemu: line 110: write error: Broken pipe
>>>>     +./common.qemu: line 110: write error: Broken pipe
>>>>     +./common.qemu: line 110: write error: Broken pipe
>>>>     +Timeout waiting for completed on handle 0
>>>>
>>>> Signed-off-by: QingFeng Hao <address@hidden>
>>>> ---
>>>>  migration/vmstate.c | 8 ++++++++
>>>>  1 file changed, 8 insertions(+)
>>>>
>>>> diff --git a/migration/vmstate.c b/migration/vmstate.c
>>>> index 78b3cd4..ff28dde 100644
>>>> --- a/migration/vmstate.c
>>>> +++ b/migration/vmstate.c
>>>> @@ -106,6 +106,10 @@ int vmstate_load_state(QEMUFile *f, const 
>>>> VMStateDescription *vmsd,
>>>>              int i, n_elems = vmstate_n_elems(opaque, field);
>>>>              int size = vmstate_size(opaque, field);
>>>>  
>>>> +            if (size == 0) {
>>>> +                field++;
>>>> +                continue;
>>>> +            }
>>>>              vmstate_handle_alloc(first_elem, field, opaque);
>>>>              if (field->flags & VMS_POINTER) {
>>>>                  first_elem = *(void **)first_elem;
>>>> @@ -322,6 +326,10 @@ void vmstate_save_state(QEMUFile *f, const 
>>>> VMStateDescription *vmsd,
>>>>              int64_t old_offset, written_bytes;
>>>>              QJSON *vmdesc_loop = vmdesc;
>>>>  
>>>> +            if (size == 0) {
>>>> +                field++;
>>>> +                continue;
>>>> +            }
>>>>              trace_vmstate_save_state_loop(vmsd->name, field->name, 
>>>> n_elems);
>>>>              if (field->flags & VMS_POINTER) {
>>>>                  first_elem = *(void **)first_elem;
>>>
>>> This is really a live migration fix, so I'm adding Juan and Dave to CC.
>>
>> You are right, this is migration stuff and has very little to do with
>> qemu-block.
>>>
>>> I suspect the real question is why a field with size 0 was even stored
>>> in the vmstate to begin with.
>>>
>>
>> I have looked onto the issue. It affects s390x only if we
>> are running without KVM. Basically, S390CPU.irqstate is unused
>> if we do not use KVM, and thus no buffer is allocated.
>>
>> IMHO this is a missing field and the cleaner way to handle such
>> missing fields is exist. However this used to work, and I recommended
>> QuiFeng Hao to discuss the problem upstream.
>>
>> By the way, I think, if we want to go back to the old behavior
>> and support VMS_VBUFFER with size 0 and nullptr, a much
>> cleaner way to do the fix is to change the assert to:
>>
>> assert(first_elem  || !n_elems || !size)
>>
>> Obviously the other clean way to fix is to implement exists.
> 
> If you're right that this specific vmstate was valid in earlier
> versions, then I think it's clear that we need to make it work again.
> Otherwise we're breaking migration from old versions.

Not really. We would not break migration because nothing was written to
the stream for VMS_VBUFFER of size 0 except the vmdesc which is at the end,
'debug only', and does not affect migration compatibility. 

IMHO it is an API question. I would have said, there is no data, therefore
there is no field if it's from scratch. But with prior history, I agree
with Dave, we should restore old behavior -- which was changed unintentionally
because I made a wrong assumption.

Regards,
Halil