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

[Max Reitz <mreitz@redhat.com>]

On 2014-10-20 at 16:48, Kevin Wolf wrote:
> Am 20.10.2014 um 16:39 hat Max Reitz geschrieben:
>> On 20.10.2014 at 16:25, Kevin Wolf wrote:
>>> Am 29.08.2014 um 23:40 hat Max Reitz geschrieben:
>>>> 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);
>>>> +    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;
>>> Might just be me, but size sounds to me as if the unit were bytes. Would
>>> you mind renaming this as refcount_block_entries or refblock_entries?
>> If I'm doing a v7, no. Otherwise, well, see l1_size and l2_size. ;-)
> Good point. This has confused people more than once, so it's probably
> not just me.

Okay, now that I've done it and was about to send the series and just 
wanted to convert a local variable named refcount_table_size to 
refcount_table_entries, I decided not do do this. I'll call it 
refcount_block_size in v7 as well.

The reason for this is that I started looking for "_size" in 
block/qcow2-refcount.c. "_entries" is never used, the number of entries 
per L1, L2 or refcount table is always foo_size. In BDRVQcowState, 
there's not only l1_size and l2_size, but refcount_table_size as well. 
Calling it refcount_block_entries without renaming those would be 
extremely weird, and renaming those does not seem like a viable option 
to me. Furthermore, I'd find it extremely confusing to have "_entries" 
in some places and "_size" in others when there's no difference between 
the two. Currently, people ask "Why is this foo_size an entry count? ... 
Well, okay, that seems to be just the way it is." With foo_entries, 
it'll be "Why is this foo_size an entry count when bar_entries exists, 
so shouldn't it be foo_entries if they want it to be an entry count?"

I'll keep it as refcount_block_size, although I'm afraid reverting all 
these changes will be as hard as having made them in the first place... 
Oh, well, here goes.

Max