On 12/11/16 05:13, Greg Kurz wrote:
> On Tue, 8 Nov 2016 16:31:08 +1100
> David Gibson <da...@gibson.dropbear.id.au> wrote:
>
>> On Fri, Nov 04, 2016 at 04:52:39PM +1100, Alexey Kardashevskiy wrote:
>>> 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 <da...@gibson.dropbear.id.au>
>>>> ---
>>>> 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:
>>>
>>> 25055@1478238734.537761:vmstate_load_field_error field "env.msr_mask" load
>>> failed, ret = -22
>>
>> Again, I don't think we generally support backwards migration.
>>
>> That said, it would be nice here to do a "set to this field on
>> ourgoing migration, but ignore on incoming migration". Do you know a
>> way to do that?
>>
>
> This doesn't exist in vmstate but it is certainly doable. An alternative
> would be to always send the fields, but have the destination to use
> pre_load and post_load callbacks to preserve its state.
A simpler way would be:
1. add a copy for each field (s/msr_mask/mig_msr_mask/),
2. get rid of _EQUAL bits,
3. implement .pre_save callback which would initialize mig_xxx
And that's it, .pre_load/.post_load is not needed.
>
>> a
>>>
>>>
>>>
>>>> VMSTATE_END_OF_LIST()
>>>> },
>>>> .subsections = (const VMStateDescription*[]) {
>>>>
>>>
>>>
>>
>
--
Alexey
signature.asc
Description: OpenPGP digital signature
