Re: [Qemu-devel] [PATCH v5 01/11] qcow2: Calculate refcount block entry count

[Max Reitz <mreitz@redhat.com>]

Am 10.10.2014 um 14:29 schrieb Benoît Canet:
> On Fri, Aug 29, 2014 at 11:40:53PM +0200, Max Reitz wrote:
>> The size of a refblock entry is (in theory) variable; calculate
>> therefore the number of entries per refblock and the according bit shift
>> (1 << x == entry count) when opening an image.
>>
>> Signed-off-by: Max Reitz <mreitz@redhat.com>
>> ---
>>   block/qcow2.c | 2 ++
>>   block/qcow2.h | 2 ++
>>   2 files changed, 4 insertions(+)
>>
>> diff --git a/block/qcow2.c b/block/qcow2.c
>> index f9e045f..172ad00 100644
>> --- a/block/qcow2.c
>> +++ b/block/qcow2.c
>> @@ -689,6 +689,8 @@ static int qcow2_open(BlockDriverState *bs, QDict *options, int flags,
>>   
>>       s->l2_bits = s->cluster_bits - 3; /* L2 is always one cluster */
>>       s->l2_size = 1 << s->l2_bits;
>> +    s->refcount_block_bits = s->cluster_bits - (s->refcount_order - 3);
> After carefull examination (s->refcount_order - 3) == REFCOUNT_SHIFT.
> Why not also creating s->refcount_shift and make use of it in order to avoid
> torturing the mind of the next reader.

I'm sorry. *g*

Well, I'm against creating s->refcount_shift, because that name implies 
you could use it for shifts. But you shouldn't do that, because it may 
be both negative and positive.

> Or simply recall in a comment that there is 2^3 bits in a byte.

I like this more. :-)

Max

> Best regards
>
> Benoît
>
>> +    s->refcount_block_size = 1 << s->refcount_block_bits;
>>       bs->total_sectors = header.size / 512;
>>       s->csize_shift = (62 - (s->cluster_bits - 8));
>>       s->csize_mask = (1 << (s->cluster_bits - 8)) - 1;
>> diff --git a/block/qcow2.h b/block/qcow2.h
>> index 6aeb7ea..7c01fb7 100644
>> --- a/block/qcow2.h
>> +++ b/block/qcow2.h
>> @@ -222,6 +222,8 @@ typedef struct BDRVQcowState {
>>       int l2_size;
>>       int l1_size;
>>       int l1_vm_state_index;
>> +    int refcount_block_bits;
>> +    int refcount_block_size;
>>       int csize_shift;
>>       int csize_mask;
>>       uint64_t cluster_offset_mask;
>> -- 
>> 2.1.0
>>