Re: [Qemu-devel] [PATCH 1/3] target/ppc: always set PPC_MEM_TLBIE in pre 2.8 migration hack

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, May 03, 2018 at 11:16:29PM +0200, Greg Kurz wrote:
> The pseries-2.7 and older machine types require CPUPPCState::insns_flags
> to be strictly equal between source and destination. This checking is
> abusive and breaks migration of KVM guests when the host CPU models
> are different, even if they are compatible enough to allow the guest
> to run transparently. This buggy behaviour was fixed for pseries-2.8
> and we added some hacks to allow backward migration of older machine
> types. These hacks assume that the CPU belongs to the POWER8 family,
> which was true for most KVM based setup we cared about at the time.
> But now POWER9 systems are coming, and backward migration of pre 2.8
> guests running in POWER8 architected mode from a POWER9 host to a
> POWER8 host is broken:
> 
> qemu-system-ppc64: error while loading state for instance 0x0 of device
>  'cpu'
> qemu-system-ppc64: load of migration failed: Invalid argument
> 
> This happens because POWER9 doesn't set PPC_MEM_TLBIE in insns_flags,
> while POWER8 does. Let's force PPC_MEM_TLBIE in the migration hack to
> fix the issue. This is an acceptable hack because these old machine
> types only support CPU models that do set PPC_MEM_TLBIE.
> 
> Signed-off-by: Greg Kurz <groug@kaod.org>
> ---
>  target/ppc/machine.c |    5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/target/ppc/machine.c b/target/ppc/machine.c
> index 3d6434a006fa..ba1b9e531f97 100644
> --- a/target/ppc/machine.c
> +++ b/target/ppc/machine.c
> @@ -212,6 +212,11 @@ static int cpu_pre_save(void *opaque)
>              ;
>          cpu->mig_msr_mask = env->msr_mask & ~metamask;
>          cpu->mig_insns_flags = env->insns_flags & insns_compat_mask;
> +        /* CPU models supported by old machines all have PPC_MEM_TLBIE,
> +         * so we set it unconditionally to allow backward migration from
> +         * a POWER9 host to a POWER8 host.

So.. the comment's slightly misleading.  This code is common to all
cpus, not just those used on pseries, so across that set it's not
really accurate.  But pseries is the only machine we support migration
to/from old versions with so it doesn't really matter.

> +         */
> +        cpu->mig_insns_flags |= PPC_MEM_TLBIE;
>          cpu->mig_insns_flags2 = env->insns_flags2 & insns_compat_mask2;
>          cpu->mig_nb_BATs = env->nb_BATs;
>      }
> 

-- 
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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