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

[Nikunj A Dadhania]

Benjamin Herrenschmidt <address@hidden> writes:

> On Thu, 2016-09-22 at 11:45 +0530, Bharata B Rao wrote:
>> On Thu, Sep 22, 2016 at 04:07:21PM +1000, Benjamin Herrenschmidt wrote:
>> > 
>> > On Thu, 2016-09-22 at 10:51 +0530, Bharata B Rao wrote:
>> > > 
>> > > 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.
>> > 
>> > Does this affect KVM ? (And if yes why on earth would KVM give a flying
>> > f*** about the TCG instruction flags ?) ... If not, then I think we can
>> > safely not care.
>> 
>> Yes, KVM migration is broken.
>
> Argh then ... stupid design in QEMU. We can't fix anything without
> breaking migration, yay !

Looking back in the history of the code:

commit: a90db1584a00dc1d1439dc7729d99674b666b85e (target-ppc: Convert
ppc cpu savevm to VMStateDescription) added this:

+        /* Sanity checking */
+        VMSTATE_UINT64_EQUAL(env.insns_flags, PowerPCCPU),
+        VMSTATE_UINT64_EQUAL(env.insns_flags2, PowerPCCPU),

These flags weren't part of vmstate, I am not sure what was the reason
behind adding it though. Its a bit old, Alexey do you remember?

> I don't know what to do to fix that to be honest. Do we have a way to filter
> what flags actually matter and filter things out when KVM is enabled ?

Something like this works for KVM:

diff --git a/target-ppc/machine.c b/target-ppc/machine.c
index 4820f22..1cf3779 100644
--- a/target-ppc/machine.c
+++ b/target-ppc/machine.c
@@ -563,8 +563,8 @@ const VMStateDescription vmstate_ppc_cpu = {
 
         /* Sanity checking */
         VMSTATE_UINTTL_EQUAL(env.msr_mask, PowerPCCPU),
-        VMSTATE_UINT64_EQUAL(env.insns_flags, PowerPCCPU),
-        VMSTATE_UINT64_EQUAL(env.insns_flags2, PowerPCCPU),
+        VMSTATE_UNUSED(sizeof(target_ulong)), /* was _EQUAL(env.insns_flags) */
+        VMSTATE_UNUSED(sizeof(target_ulong)), /* was _EQUAL(env.insns_flags2) 
*/
         VMSTATE_UINT32_EQUAL(env.nb_BATs, PowerPCCPU),
         VMSTATE_END_OF_LIST()
     },

TCG migration still remains broken with this.

Regards,
Nikunj