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

[David Gibson]

On Thu, Sep 22, 2016 at 12:32:24PM +0200, Paolo Bonzini wrote:
> 
> 
> On 22/09/2016 12:04, Benjamin Herrenschmidt wrote:
> > On Thu, 2016-09-22 at 14:34 +0530, Nikunj A Dadhania wrote:
> >> 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.
> > 
> > Can we have conditionally present flags and a post-load that does some
> > matching ?
> 
> Yes, you can use something like
> 
>       VMSTATE_UINT64(env.src_insns_flags, PowerPCCPU),
>       VMSTATE_UINT64(env.src_insns_flags2, PowerPCCPU),
> 
> and a post_load that compares them if kvm_enabled() only.

We could, but I'm not convinced there's any point.  I don't see that
migrating these flags actually has any value beyond a sanity check,
the consequences of which we obviously didn't think through fully.
They should just be a TCG internal matter.

> *However* a better fix would be to preserve the old flags for
> pseries-2.6, and only set the newer flags for pseries-2.7.  I'm not
> saying you have to do this, but if it's not hard (no idea) why not learn
> how to do it right.
> 
> The design is not stupid, it's just that compatibility is harder than
> you think and you are going through the same learning experiences that
> x86 went though.
> 
> Paolo
> 

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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