Re: [Qemu-devel] [PATCH] block: Fix vpc initialization of the Dynamic Disk Header

[Charles Arnold]

>>> On 11/9/2011 at 02:46 AM, in message <address@hidden>, Kevin Wolf
<address@hidden> wrote: 
> Am 08.11.2011 20:25, schrieb Charles Arnold:
>> The Data Offset field in the Dynamic Disk Header is an 8 byte field.
>> Although the specification (2006-10-11) gives an example of initializing
>> only the first 4 bytes, images generated by Microsoft on Windows initialize 
>> all 8 bytes.
>> 
>> Failure to initialize all 8 bytes results in errors from utilities
>> that check specifically for the complete Data Offset field initialization.
>> 
>> Signed-off-by: Charles Arnold <address@hidden>
>> 
>> diff --git a/block/vpc.c b/block/vpc.c
>> index 416f489..35ac3fd 100644
>> --- a/block/vpc.c
>> +++ b/block/vpc.c
>> @@ -585,7 +585,7 @@ static int vpc_create(const char *filename, 
> QEMUOptionParameter *options)
>> 
>>      memcpy(dyndisk_header->magic, "cxsparse", 8);
>> 
>> -    dyndisk_header->data_offset = be64_to_cpu(0xFFFFFFFF);
>> +    dyndisk_header->data_offset = be64_to_cpu(0xFFFFFFFFFFFFFFFFULL);
>>      dyndisk_header->table_offset = be64_to_cpu(3 * 512);
>>      dyndisk_header->version = be32_to_cpu(0x00010000);
>>      dyndisk_header->block_size = be32_to_cpu(block_size);
> 
> Can you please add a short comment explaining why we deviate from the
> specification in this point? If someone notices the discrepancy later,
> he shouldn't have to dig up the git commit message.
> 
> Kevin
 
Using qemu-img to convert a raw image to a vhd image seems to work fine accept 
that only 4 bytes of the 8 byte Dynamic Disk Header Data Offset field are 
initialized.  This seems correct according to the specification but tools like 
Citrix's vhd-util fail when querying the image due to the shortened 
initialization.  Upon further analysis, it was determined that Microsoft also 
initializes all 8 bytes when generating a vhd image on windows.  Initializing 
all 8 bytes seems more logical and may indicate that Microsoft needs to update 
the specification because the requirement for this field changed or that it is 
simply an oversight that has never been noticed.

- Charles