Re: [Qemu-devel] [PATCH v2 10/17] block: define get_block_status return value

[Paolo Bonzini]

Il 19/07/2013 08:48, Peter Lieven ha scritto:
>>
>> -    return bdrv_get_block_status(bs, sector_num, nb_sectors, pnum);
>> +    int64_t ret = bdrv_get_block_status(bs, sector_num, nb_sectors,
>> pnum);
>> +    return
>> +        (ret & BDRV_BLOCK_DATA) ||
>> +        ((ret & BDRV_BLOCK_ZERO) && !bdrv_has_zero_init(bs));
> 
> i do also not understand the "((ret & BDRV_BLOCK_ZERO) &&
> !bdrv_has_zero_init(bs))";
> if a block is unallocated and reads as zero, but the device lacks zero
> init, it is declared as allocated with this, isn't it?

Thinking more about it, I would say it is a bugfix.

If a block is unallocated and reads as zero, but the device lacks zero
init, the block contents have changed since the guest was created.  Thus
the guest might well be relying on the zero content of the block, and it
should be treated as allocated.

In any case, better safe than sorry---if in doubt, the conservative
answer is always to return 1, and callers who want more precise
information can use bdrv_get_block_status.

Paolo
> for iscsi and host_device with lbprz==1 or discardzeroes respectively all
> blocks would return as allocated. is this wanted?