Re: [Qemu-devel] [PATCH 05/16] block/parallels: support padded Parallels images

["Denis V. Lunev" <den@openvz.org>]

On 15/12/14 14:05, Kevin Wolf wrote:
> Am 15.12.2014 um 09:27 hat Denis V. Lunev geschrieben:
>> Unfortunately, old guest OSes do not align partitions to page size by
>> default. This is true for Windows 2003 and Windows XP.
>>
>> For the time being Parallels was created an optimization for such OSes
>> in its desktop product. Desktop users are not qualified enough to create
>> properly aligned installations. Thus Parallels makes a blind guess
>> on a customer behalf and creates so-called "padded" images if guest
>> OS type is specified as WinXP, Win2k and Win2k3.
>>
>> "Padding" is a value which should be added to guest LBA to obtain
>> sector number inside the image. This results in a shifted images.
>>     0123        offset inside image (in 512 byte sectors)
>>    +-------
>>    +.012        guest data (512 byte sectors)
>>    +-------
>> The information about this is available in DiskDescriptor.xml ONLY. There
>> is no such data in the image header.
>>
>> There share of such images could be evaluated as 6-8% according to the
>> statistics in my hands.
>>
>> This patch obtains proper value from XML and applies it on reading.
>>
>> Signed-off-by: Denis V. Lunev <den@openvz.org>
>> Acked-by: Roman Kagan <rkagan@parallels.com>
>> Reviewed-by: Jeff Cody <jcody@redhat.com>
>> CC: Kevin Wolf <kwolf@redhat.com>
>> CC: Stefan Hajnoczi <stefanha@redhat.com>
>> ---
>>   block/parallels.c | 19 +++++++++++++++++++
>>   1 file changed, 19 insertions(+)
>>
>> diff --git a/block/parallels.c b/block/parallels.c
>> index c22b91b..fedb009 100644
>> --- a/block/parallels.c
>> +++ b/block/parallels.c
>> @@ -63,6 +63,7 @@ typedef struct BDRVParallelsState {
>>       unsigned int tracks;
>>   
>>       unsigned int off_multiplier;
>> +    unsigned int padding;
>>   } BDRVParallelsState;
>>   
>>   
>> @@ -235,6 +236,7 @@ static int parallels_open_xml(BlockDriverState *bs, int flags, Error **errp)
>>       const char *data;
>>       char image_path[PATH_MAX];
>>       Error *local_err = NULL;
>> +    BDRVParallelsState *s = bs->opaque;
>>   
>>       ret = size = bdrv_getlength(bs->file);
>>       if (ret < 0) {
>> @@ -264,6 +266,19 @@ static int parallels_open_xml(BlockDriverState *bs, int flags, Error **errp)
>>       if (root == NULL) {
>>           goto fail;
>>       }
>> +
>> +    data = xml_get_text(root, "Disk_Parameters", "Padding", NULL);
>> +    if (data != NULL) {
>> +        char *endptr;
>> +        unsigned long pad;
>> +
>> +        pad = strtoul(data, &endptr, 0);
>> +        if ((endptr != NULL && *endptr != '\0') || pad > UINT_MAX) {
> Can endptr even be NULL? Also, shouldn't you set errno = 0 before and
> check it here?
got it, I think you are right
>> +            goto fail;
>> +        }
>> +        s->padding = (uint32_t)pad;
> s->padding is unsigned int, pad is unsigned long. Why the cast to
> uint32_t here, which is different from both?
ok
>> +    }
>> +
>>       image = xml_seek(root, "StorageData", "Storage", "Image", NULL);
>>       data = ""; /* make gcc happy */
>>       for (size = 0; image != NULL; image = image->next) {
>> @@ -365,6 +380,10 @@ static int64_t seek_to_sector(BlockDriverState *bs, int64_t sector_num)
>>   static int parallels_read(BlockDriverState *bs, int64_t sector_num,
>>                       uint8_t *buf, int nb_sectors)
>>   {
>> +    BDRVParallelsState *s = bs->opaque;
>> +
>> +    sector_num += s->padding;
> No check needed here? bdrv_check_request() has checked sector_num and
> nb_sectors against the image size, but now you can't rely on the result
> any more.
you are perfectly correct. I'll recheck the situation.
I have seen some dances with image size. Allowed
image size should be extended with 1 block, but
I am not quite sure.

Thank you for pointing this out.

>>       while (nb_sectors > 0) {
>>           int64_t position = seek_to_sector(bs, sector_num);
>>           if (position >= 0) {
> Kevin