Re: [Qemu-devel] [RFC 16/17] ppc: Remove counter-productive "sanity checks" in migration

[Alexey Kardashevskiy]

On 30/10/16 22:12, David Gibson wrote:
> When vmstate for the ppc cpu was introduced in a90db158 "target-ppc:
> Convert ppc cpu savevm to VMStateDescription", several "sanity check"
> fields were included, verifying that certain cpu parameters matched between
> source and destination.
> 
> This turns out not to have been a good idea.  For one thing it's redundant
> with existing checks for a compatible cpu version at either end.  But more
> importantly the insns_flags and insns_flags2 checks actively break things:
> they expose what's essentially an internal TCG implementation detail in the
> migration stream.  That means that when new instruction classes are added
> or rearranged, migration can break.
> 
> This removes these ill-considered sanity checks.
> 
> Signed-off-by: David Gibson <address@hidden>
> ---
>  target-ppc/machine.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/target-ppc/machine.c b/target-ppc/machine.c
> index 62b9e94..453ef0a 100644
> --- a/target-ppc/machine.c
> +++ b/target-ppc/machine.c
> @@ -602,10 +602,10 @@ const VMStateDescription vmstate_ppc_cpu = {
>          /* FIXME: access_type? */
>  
>          /* Sanity checking */
> -        VMSTATE_UINTTL_EQUAL(env.msr_mask, PowerPCCPU),
> -        VMSTATE_UINT64_EQUAL(env.insns_flags, PowerPCCPU),
> -        VMSTATE_UINT64_EQUAL(env.insns_flags2, PowerPCCPU),
> -        VMSTATE_UINT32_EQUAL(env.nb_BATs, PowerPCCPU),
> +        VMSTATE_UNUSED(sizeof(target_ulong) /* msr_mask */
> +                       + sizeof(uint64_t) /* insns_flags */
> +                       + sizeof(uint64_t) /* insns_flags2 */
> +                       + sizeof(uint32_t)), /* nb_BATs */


This breaks migration to older QEMU:

address@hidden:vmstate_load_field_error field "env.msr_mask" load
failed, ret = -22



>          VMSTATE_END_OF_LIST()
>      },
>      .subsections = (const VMStateDescription*[]) {
> 


-- 
Alexey