Re: [PATCH v8 21/21] libxl: add support for migrating HVM guests without a device model

["Roger Pau Monné" <roger.pau@citrix.com>]

El 11/11/15 a les 18.14, Andrew Cooper ha escrit:
> On 06/11/15 16:05, Roger Pau Monne wrote:
>> diff --git a/tools/libxl/libxl_stream_write.c b/tools/libxl/libxl_stream_write.c
>> index 52a60d7..0a6eaf9 100644
>> --- a/tools/libxl/libxl_stream_write.c
>> +++ b/tools/libxl/libxl_stream_write.c
>> @@ -232,6 +232,9 @@ void libxl__stream_write_start(libxl__egc *egc,
>>              stream->emu_sub_hdr.id = EMULATOR_QEMU_UPSTREAM;
>>              break;
>>  
>> +        case LIBXL_DEVICE_MODEL_VERSION_NONE:
>> +            break;
>> +
> 
> This (in principle) leaves stream->emu_sub_hdr.id uninitialised
> (although its value will be zero because of libxl__stream_write_init()).
> 
> I would be tempted to (ab)use EMULATOR_UNKNOWN here and have
> setup_emulator_write() assert id != UNKNOWN, to catch calls which slip
> through the cracks.

This seems fine to me and I've implemented it.

> 
>>          default:
>>              rc = ERROR_FAIL;
>>              LOG(ERROR, "Unknown emulator for HVM domain");
>> @@ -359,6 +362,12 @@ static void write_emulator_xenstore_record(libxl__egc *egc,
>>      char *buf = NULL;
>>      uint32_t len = 0;
>>  
>> +    if (libxl__device_model_version_running(gc, dss->domid) ==
>> +        LIBXL_DEVICE_MODEL_VERSION_NONE) {
>> +        emulator_xenstore_record_done(egc, stream);
>> +        return;
>> +    }
>> +
> 
> libxl__device_model_version_running() performs a xenstore action, which
> is a steep overhead for constant information.
> 
> It would be better to introduce a new field in libxl__stream_write_state
> and cache libxl__device_model_version_running() once in
> libxl__stream_write_start().  You can then also use that for an
> assertion in setup_emulator_write()

Right, so I've ended up adding two asserts in setup_emulator_write:

assert(stream->emu_sub_hdr.id != EMULATOR_UNKNOWN);
assert(stream->device_model_version != LIBXL_DEVICE_MODEL_VERSION_NONE);

Roger.