Re: [Qemu-devel] [PATCH v2] Consolidate reads and writes in nbd block device into one common routine

[Michael Tokarev <mjt@tls.msk.ru>]

On 28.02.2012 17:03, Paolo Bonzini wrote:
> Il 28/02/2012 13:35, Michael Tokarev ha scritto:
>> On 28.02.2012 15:35, Paolo Bonzini wrote:
>>> Il 28/02/2012 11:24, Michael Tokarev ha scritto:
>>>> This removes quite some duplicated code.
>> []
>>>> +static int nbd_co_rwv(BlockDriverState *bs, int64_t sector_num,
>>>> +                      int nb_sectors, QEMUIOVector *qiov, int iswrite)
>>>
>>> Call this nbd_co_rw, and please pass the whole request.type down.
>>
>> Originally it is readV and writeV, so why it should not be rwV ?
> 
> It's more consistent with block.c.
> 
>> By passing whole request.type (NBD_CMD_WRITE or NBD_CMD_WRITE|NBD_CMD_FLAG_FUA
>> or NBD_CMD_READ) the condition (iswrite currently) will be larger (request.type
>> != NBD_CMD_READ).  Also, if someday we'll have additional flag for READ as we
>> already do for write, whole thing will be even more difficult to read.
> 
> Sure, but why should a generic function deal with NBD_CMD_FLAG_FUA?

I can pass both "iswrite" and request.type here - to avoid possible
complications in the area of adding more nbd-specific meanings/flags
to request.type.  But that becomes more ugly.

[]
>>> ... but thinking more about it, why don't you leave
>>> nbd_co_readv_1/nbd_co_writev_1 alone, and create a nbd_split_rw function
>>> that takes a function pointer?
>>
>> Because each of these nbd_co_*_1 does the same thing, the diff. is
>> only quiv->iov vs NULL.  While reading the original code it was the
>> first thing I did - consolidated nbd_co_*_1 into nbd_co_* ;)
> 
> And offset.  I needed to check that non-0 offsets are fine when the iov
> is NULL; it's not obvious.

It isn't indeed.  Both send_request and recv_reply only checks iov
and ignores offset if iov is NULL.

>> Actually it might be a good idea to have single bdrv->bdrv_co_readwritev
>> method instead of two -- the path of each read and write jumps between
>> specific read-or-write routine and common readwrite routine several
>> times.
> 
> Small amounts of duplicate code can be better than functions with many
> ifs or complicated conditions.

Sure thing.  But when the code path is so twisted - common->specific->
common-> specific - it makes very difficult to get the bigger picture.

>> I see only one correction which needs (really!) to be done - that's
>> fixing the bug with offset.  Do you still not agree?
> 
> I still disagree. :)  I will accept the patch with the function pointer
> though.

With separate nbd_co_*_1 it isn't worth the effort.  When it all is in a
_small_ single routine (the resulting function is actually very small),
whole logic is immediately visible.  In particular, the FUA bit which
is set for every _part_ of write request - it wasn't visible till I
integrated nbd_co_writev_1 into nbd_co_writev.

Thanks,

/mjt