Re: [Qemu-ppc] [RFC 12/17] ppc: Migrate compatibility mode

[Alexey Kardashevskiy]

On 08/11/16 16:19, David Gibson wrote:
> On Fri, Nov 04, 2016 at 04:58:47PM +1100, Alexey Kardashevskiy wrote:
>> On 30/10/16 22:12, David Gibson wrote:
>>> Server-class POWER CPUs can be put into several compatibility modes.  These
>>> can be specified on the command line, or negotiated by the guest during
>>> boot.
>>>
>>> Currently we don't migrate the compatibility mode, which means after a
>>> migration the guest will revert to running with whatever compatibility
>>> mode (or none) specified on the command line.
>>>
>>> With the limited range of CPUs currently used, this doesn't usually cause
>>> a problem, but it could.  Fix this by adding the compatibility mode (if
>>> set) to the migration stream.
>>>
>>> Signed-off-by: David Gibson <address@hidden>
>>> ---
>>>  target-ppc/machine.c | 34 ++++++++++++++++++++++++++++++++++
>>>  1 file changed, 34 insertions(+)
>>>
>>> diff --git a/target-ppc/machine.c b/target-ppc/machine.c
>>> index 4820f22..5d87ff6 100644
>>> --- a/target-ppc/machine.c
>>> +++ b/target-ppc/machine.c
>>> @@ -9,6 +9,7 @@
>>>  #include "mmu-hash64.h"
>>>  #include "migration/cpu.h"
>>>  #include "exec/exec-all.h"
>>> +#include "qapi/error.h"
>>>  
>>>  static int cpu_load_old(QEMUFile *f, void *opaque, int version_id)
>>>  {
>>> @@ -176,6 +177,20 @@ static int cpu_post_load(void *opaque, int version_id)
>>>       * software has to take care of running QEMU in a compatible mode.
>>>       */
>>>      env->spr[SPR_PVR] = env->spr_cb[SPR_PVR].default_value;
>>> +
>>> +#if defined(TARGET_PPC64)
>>> +    if (cpu->compat_pvr) {
>>> +        Error *local_err = NULL;
>>> +
>>> +        ppc_set_compat(cpu, cpu->compat_pvr, &local_err);
>>> +        if (local_err) {
>>> +            error_report_err(local_err);
>>> +            error_free(local_err);
>>> +            return -1;
>>> +        }
>>> +    }
>>> +#endif
>>> +
>>>      env->lr = env->spr[SPR_LR];
>>>      env->ctr = env->spr[SPR_CTR];
>>>      cpu_write_xer(env, env->spr[SPR_XER]);
>>> @@ -528,6 +543,24 @@ static const VMStateDescription vmstate_tlbmas = {
>>>      }
>>>  };
>>>  
>>> +static bool compat_needed(void *opaque)
>>> +{
>>> +    PowerPCCPU *cpu = opaque;
>>> +
>>> +    return cpu->compat_pvr != 0;
>>
>>
>> Finally got to trying how this affects migration :)
>>
>> This breaks migration to QEMU <=2.7, and it should not at least when both
>> source and destination are running with  -cpu host,compat=power7.
> 
> IIUC, we don't generally try to maintain backwards migration, even for
> old machine types.


I thought the opposite - we generally try to maintain it, this is pretty
much why we use these subsections in cases like this; otherwise you could
just add a new field and bump the vmstate_ppc_cpu.version.


> 
>>
>>
>>> +}
>>> +
>>> +static const VMStateDescription vmstate_compat = {
>>> +    .name = "cpu/compat",
>>> +    .version_id = 1,
>>> +    .minimum_version_id = 1,
>>> +    .needed = compat_needed,
>>> +    .fields = (VMStateField[]) {
>>> +        VMSTATE_UINT32(compat_pvr, PowerPCCPU),
>>> +        VMSTATE_END_OF_LIST()
>>> +    }
>>> +};
>>> +
>>>  const VMStateDescription vmstate_ppc_cpu = {
>>>      .name = "cpu",
>>>      .version_id = 5,
>>> @@ -580,6 +613,7 @@ const VMStateDescription vmstate_ppc_cpu = {
>>>          &vmstate_tlb6xx,
>>>          &vmstate_tlbemb,
>>>          &vmstate_tlbmas,
>>> +        &vmstate_compat,
>>>          NULL
>>>      }
>>>  };
>>>
>>
>>
> 


-- 
Alexey

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