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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Bharata B Rao (bharata@linux.vnet.ibm.com) 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.

Dave

> Regards,
> Bharata.
> 
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK