Re: [Qemu-devel] [PATCH 2/2] spapr_nvram: Enable migration

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 09/25/2014 07:43 PM, Alexander Graf wrote:
> 
> 
> On 25.09.14 09:02, Alexey Kardashevskiy wrote:
>> The only case when sPAPR NVRAM migrates now is if is backed by a file and
>> copy-storage migration is performed.
>>
>> This enables RAM copy of NVRAM even if NVRAM is backed by a file.
>>
>> This defines a VMSTATE descriptor for NVRAM device so the memory copy
>> of NVRAM can migrate and be written to a backing file on the destination
>> if one is provided.
>>
>> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
>> ---
>>  hw/nvram/spapr_nvram.c | 68 +++++++++++++++++++++++++++++++++++++++++++-------
>>  1 file changed, 59 insertions(+), 9 deletions(-)
>>
>> diff --git a/hw/nvram/spapr_nvram.c b/hw/nvram/spapr_nvram.c
>> index 6a72ef4..254009e 100644
>> --- a/hw/nvram/spapr_nvram.c
>> +++ b/hw/nvram/spapr_nvram.c
>> @@ -76,15 +76,20 @@ static void rtas_nvram_fetch(PowerPCCPU *cpu, sPAPREnvironment *spapr,
>>          return;
>>      }
>>  
>> +    assert(nvram->buf);
>> +
>>      membuf = cpu_physical_memory_map(buffer, &len, 1);
>> +
>> +    alen = len;
>>      if (nvram->drive) {
>>          alen = bdrv_pread(nvram->drive, offset, membuf, len);
>> +        if (alen > 0) {
>> +            memcpy(nvram->buf + offset, membuf, alen);
> 
> Why?

This way I do not need pre_save hook and I keep the buf in sync with the
file. If I implement pre_save, then buf will serve 2 purposes - it is
either NVRAM itself (if there is no backing file, exists during guest's
lifetime) or it is a migration copy (exists between pre_save and post_load
and then it is disposed). Two quite different uses of the same thing
confuse me. But - I do not mind doing it your way, no big deal, should I?


>> +        }
>>      } else {
>> -        assert(nvram->buf);
>> -
>>          memcpy(membuf, nvram->buf + offset, len);
>> -        alen = len;
>>      }
>> +
>>      cpu_physical_memory_unmap(membuf, len, 1, len);
>>  
>>      rtas_st(rets, 0, (alen < len) ? RTAS_OUT_HW_ERROR : RTAS_OUT_SUCCESS);
>> @@ -122,14 +127,15 @@ static void rtas_nvram_store(PowerPCCPU *cpu, sPAPREnvironment *spapr,
>>      }
>>  
>>      membuf = cpu_physical_memory_map(buffer, &len, 0);
>> +
>> +    alen = len;
>>      if (nvram->drive) {
>>          alen = bdrv_pwrite(nvram->drive, offset, membuf, len);
>> -    } else {
>> -        assert(nvram->buf);
>> -
>> -        memcpy(nvram->buf + offset, membuf, len);
>> -        alen = len;
>>      }
>> +
>> +    assert(nvram->buf);
>> +    memcpy(nvram->buf + offset, membuf, len);
>> +
>>      cpu_physical_memory_unmap(membuf, len, 0, len);
>>  
>>      rtas_st(rets, 0, (alen < len) ? RTAS_OUT_HW_ERROR : RTAS_OUT_SUCCESS);
>> @@ -144,9 +150,10 @@ static int spapr_nvram_init(VIOsPAPRDevice *dev)
>>          nvram->size = bdrv_getlength(nvram->drive);
>>      } else {
>>          nvram->size = DEFAULT_NVRAM_SIZE;
>> -        nvram->buf = g_malloc0(nvram->size);
>>      }
>>  
>> +    nvram->buf = g_malloc0(nvram->size);
>> +
>>      if ((nvram->size < MIN_NVRAM_SIZE) || (nvram->size > MAX_NVRAM_SIZE)) {
>>          fprintf(stderr, "spapr-nvram must be between %d and %d bytes in size\n",
>>                  MIN_NVRAM_SIZE, MAX_NVRAM_SIZE);
>> @@ -166,6 +173,48 @@ static int spapr_nvram_devnode(VIOsPAPRDevice *dev, void *fdt, int node_off)
>>      return fdt_setprop_cell(fdt, node_off, "#bytes", nvram->size);
>>  }
>>  
>> +static int spapr_nvram_pre_load(void *opaque)
>> +{
>> +    sPAPRNVRAM *nvram = VIO_SPAPR_NVRAM(opaque);
>> +
>> +    g_free(nvram->buf);
>> +    nvram->buf = NULL;
>> +    nvram->size = 0;
>> +
>> +    return 0;
>> +}
>> +
>> +static int spapr_nvram_post_load(void *opaque, int version_id)
>> +{
>> +    sPAPRNVRAM *nvram = VIO_SPAPR_NVRAM(opaque);
>> +
>> +    if (nvram->drive) {
>> +        int alen = bdrv_pwrite(nvram->drive, 0, nvram->buf, nvram->size);
> 
> In the file backed case you're already overwriting the disk backed
> version, no?

Yes. Is that bad?


> Also, couldn't you just do the copy and provisioning of buf in a
> pre_save hook?


I can do this too. I just do not see why that would be lot better though :)



-- 
Alexey