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

[Michael Tokarev]

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 ?

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.

> 
>>  {
>>      BDRVNBDState *s = bs->opaque;
>>      struct nbd_request request;
>>      struct nbd_reply reply;
>> +    int offset = 0;
>>  
>> -    request.type = NBD_CMD_WRITE;
>> -    if (!bdrv_enable_write_cache(bs) && (s->nbdflags & NBD_FLAG_SEND_FUA)) {
>> +    request.type = iswrite ? NBD_CMD_WRITE : NBD_CMD_READ;
>> +    if (iswrite && !bdrv_enable_write_cache(bs) && (s->nbdflags & 
>> NBD_FLAG_SEND_FUA)) {
>>          request.type |= NBD_CMD_FLAG_FUA;
>>      }
>> +    reply.error = 0;
>> +
>> +    /* we split the request into pieces of at most NBD_MAX_SECTORS size
>> +     * and process them in a loop... */
>> +    do {
>> +        request.from = sector_num * 512;
>> +        request.len = MIN(nb_sectors, NBD_MAX_SECTORS) * 512;
>> +
>> +        nbd_coroutine_start(s, &request);
>> +        if (nbd_co_send_request(s, &request, iswrite ? qiov->iov : NULL, 0) 
>> == -1) {
> 
> The last 0 needs to be offset.

Indeed, this is a bug.  I think I tested it with large requests
but it looks like only for reads.

[]
> ... 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_* ;)

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.

I see only one correction which needs (really!) to be done - that's
fixing the bug with offset.  Do you still not agree?

Thanks,

/mjt