Re: [Qemu-devel] pseries-2.6 migration from QEMU-2.6 to QEMU-2.7 broken

[Nikunj A Dadhania]

"Dr. David Alan Gilbert" <address@hidden> writes:

> * Bharata B Rao (address@hidden) wrote:
>> On Thu, Sep 22, 2016 at 03:30:08PM +1000, David Gibson wrote:
>> > On Thu, Sep 22, 2016 at 10:51:05AM +0530, Bharata B Rao wrote:
>> > > Hi,
>> > > 
>> > > Nikunj and I realized that migrating pseries-2.6 guest from QEMU-2.6
>> > > to newer QEMU-2.7 is broken like this:
>> > > 
>> > > qemu-system-ppc64: error while loading state for instance 0x0 of device 
>> > > 'cpu'
>> > > qemu-system-ppc64: load of migration failed: Invalid argument
>> > > 
>> > > Bisecting tells us that 4e0806110c8b896ceff3490f15a616e8b3165efe
>> > > (ppc: Add PPC_64H instruction flag to POWER7 and POWER8) is the
>> > > first bad commit.  Along with this there are other 3 similar commits
>> > > which add new bits to insns_flags and insns_flags2 fields of POWER7
>> > > and POWER8 CPUs.
>> > > 
>> > > 4e0806110c8b896ceff3490f15a616e8b3165efe Adds PPC_64H to POWER7 and 
>> > > POWER8
>> > > dfdd3e43620a6cd4f2be31da5a257c84a16fc000 Adds PPC_64BX to POWER7
>> > > b781537560e3b968b6fe1395e3d07bd67f0009ba Adds PPC_CILDST to POWER7 and 
>> > > POWER8
>> > > 7778a575c7055276afdd01737e9d1029a65f923d Adds PPC2_PM_ISA206 to POWER7 
>> > > and POWER8
>> > > 
>> > > The flag values are expected to remain same for a machine version for
>> > > the migration to succeed, but this expectation is broken now. Should
>> > > we make the addition of these flags conditional on machine type version ?
>> > > But these flags are part of POWER8 CPU definition which is common for
>> > > both pseries and upcoming powernv.
>> > 
>> > Can you step me through how the new flags are breaking the migration?
>> > It's not immediately obvious to me.
>> 
>> Here is what I understand. Given below is the pruned vmstate_ppc_cpu
>> data structure.
>> 
>> const VMStateDescription vmstate_ppc_cpu = {
>>     .name = "cpu",
>>     .fields = (VMStateField[]) {
>>         /* Sanity checking */
>>         VMSTATE_UINT64_EQUAL(env.insns_flags, PowerPCCPU),
>>         VMSTATE_UINT64_EQUAL(env.insns_flags2, PowerPCCPU),
>>         VMSTATE_END_OF_LIST()
>>     },
>> };
>> 
>> When pseries-2.6 guest is started at the source with QEMU-2.6, insns_flags
>> and insns_flags2 will not have PPC_64H, PPC_64BX, PPC_CILDST, PPC2_PM_ISA206
>> set. However at the target when pseries-2.6 guest is started with QEMU-2.7,
>> these flags will be set. And I believe VMSTATE_UINT64_EQUAL checks above
>> will cause migration to fail.
>
> You might find the first two patches in:
>    https://lists.gnu.org/archive/html/qemu-devel/2016-08/msg03681.html
> useful in debugging this; it prints the values when the _EQUAL macros fail 
> and prints
> the field name that fails.

Thanks, we were using trace, this is very helpful without trace
during error conditions.

qemu-system-ppc64: 9223477658187168481 != 9223477658187151905
qemu-system-ppc64: Failed to load cpu:env.insns_flags
qemu-system-ppc64: error while loading state for instance 0x0 of device 'cpu'
qemu-system-ppc64: load of migration failed: Invalid argument

Regards,
Nikunj