Re: [Qemu-devel] [PATCH v3 11/19] block: define get_block_status return value

[Paolo Bonzini <pbonzini@redhat.com>]

Il 30/07/2013 16:14, Kevin Wolf ha scritto:
> Am 25.07.2013 um 16:23 hat Paolo Bonzini geschrieben:
>> Define the return value of get_block_status.  Bits 0, 1, 2 and 9-62
>> are valid; bit 63 (the sign bit) is reserved for errors.  Bits 3-8
>> are left for future extensions.
>>
>> The return code is compatible with the old is_allocated API: if a driver
>> only returns 0 or 1 (aka BDRV_BLOCK_DATA) like is_allocated used to,
>> clients of is_allocated will not have any change in behavior.  Still,
>> we will return more precise information in the next patches and the
>> new definition of bdrv_is_allocated is already prepared for this.
>>
>> Reviewed-by: Eric Blake <eblake@redhat.com>
>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>> ---
>>  block.c               | 10 ++++++++--
>>  include/block/block.h | 26 ++++++++++++++++++++++++++
>>  2 files changed, 34 insertions(+), 2 deletions(-)
>>
>> diff --git a/block.c b/block.c
>> index f533c36..7cfbf71 100644
>> --- a/block.c
>> +++ b/block.c
>> @@ -3004,7 +3004,7 @@ static int64_t coroutine_fn bdrv_co_get_block_status(BlockDriverState *bs,
>>  
>>      if (!bs->drv->bdrv_co_get_block_status) {
>>          *pnum = nb_sectors;
>> -        return 1;
>> +        return BDRV_BLOCK_DATA;
>>      }
>>  
>>      return bs->drv->bdrv_co_get_block_status(bs, sector_num, nb_sectors, pnum);
>> @@ -3054,7 +3054,13 @@ int64_t bdrv_get_block_status(BlockDriverState *bs, int64_t sector_num,
>>  int coroutine_fn bdrv_is_allocated(BlockDriverState *bs, int64_t sector_num,
>>                                     int nb_sectors, int *pnum)
>>  {
>> -    return bdrv_get_block_status(bs, sector_num, nb_sectors, pnum);
>> +    int64_t ret = bdrv_get_block_status(bs, sector_num, nb_sectors, pnum);
>> +    if (ret < 0) {
>> +        return ret;
>> +    }
>> +    return
>> +        (ret & BDRV_BLOCK_DATA) ||
>> +        ((ret & BDRV_BLOCK_ZERO) && !bdrv_has_zero_init(bs));
>>  }
>>  
>>  /*
>> diff --git a/include/block/block.h b/include/block/block.h
>> index e41854e..d044b31 100644
>> --- a/include/block/block.h
>> +++ b/include/block/block.h
>> @@ -81,6 +81,32 @@ typedef struct BlockDevOps {
>>  #define BDRV_SECTOR_SIZE   (1ULL << BDRV_SECTOR_BITS)
>>  #define BDRV_SECTOR_MASK   ~(BDRV_SECTOR_SIZE - 1)
>>  
>> +/* BDRV_BLOCK_DATA: data is read from bs->file or another file
>> + * BDRV_BLOCK_ZERO: sectors read as zero
>> + * BDRV_BLOCK_OFFSET_VALID: sector stored in bs->file as raw data
>> + *
>> + * If BDRV_BLOCK_OFFSET_VALID is set, bits 9-62 represent the offset in
>> + * bs->file where sector data can be read from as raw data.
>> + *
>> + * DATA == 0 && ZERO == 0 means that data is read from backing_hd if present.
>> + *
>> + * DATA ZERO OFFSET_VALID
>> + *  t    t        t       sectors read as zero, bs->file is zero at offset
>> + *  t    f        t       sectors read as valid from bs->file at offset
>> + *  f    t        t       sectors preallocated, read as zero, bs->file not
>> + *                        necessarily zero at offset
>> + *  f    f        t       sectors preallocated but read from backing_hd,
>> + *                        bs->file contains garbage at offset
>> + *  t    t        f       sectors preallocated, read as zero, unknown offset
>> + *  t    f        f       sectors read from unknown file or offset
>> + *  f    t        f       not allocated or unknown offset, read as zero
>> + *  f    f        f       not allocated or unknown offset, read from backing_hd
>> + */
>> +#define BDRV_BLOCK_DATA         1
>> +#define BDRV_BLOCK_ZERO         2
>> +#define BDRV_BLOCK_OFFSET_VALID 4
>> +#define BDRV_BLOCK_OFFSET_MASK  BDRV_SECTOR_MASK
> 
> When are block driver supposed to set the BDRV_BLOCK_OFFSET_VALID flag?

Always if they can provide the information.

> For example, qcow2 could in theory set the flag, it has all of the
> information already in memory.

In fact it does later in the series.

> But with a fragmented image this might
> mean that it returns only one cluster instead of a large area with one
> bdrv_get_block_status() call.

This is just theoretical, right?  qcow2_get_cluster_offset only works on
areas that are contiguous in the raw image.

> Should the caller pass a flag that tells whether he is interested in the
> offset or not?

That would complicate the API (and the implementation too if you want to
honor it in the formats).  Since is_allocated works the same way and it
wasn't a problem so far, I decided not to have such a flag.

Paolo