Re: [Qemu-devel] [PATCH 1/2] s390x/3270: IDA support for 3270 via CcwDataStream

[Halil Pasic <pasic@linux.vnet.ibm.com>]

On 09/21/2017 11:15 AM, Cornelia Huck wrote:
>> +static inline CcwDataStream *get_cds(Terminal3270 *t)
>> +{
>> +    return &(CCW_DEVICE(&t->cdev)->sch->cds);
>> +}
>> +
>> +static int read_payload_3270(EmulatedCcw3270Device *dev)
>>  {
>>      Terminal3270 *t = TERMINAL_3270(dev);
>>      int len;
>>  
>> -    len = MIN(count, t->in_len);
>> -    cpu_physical_memory_write(cda, t->inv, len);
>> +    len = MIN(ccw_dstream_avail(get_cds(t)), t->in_len);
>> +    ccw_dstream_write_buf(get_cds(t), t->inv, len);
> CCW_DEVICE() as called by get_cds() goes through qom, which implies a
> bit of overhead. Not sure if it makes sense to cache it in this
> function so you don't go through it multiple times. (Dito for the other
> callback.)
> 

I've cargo-culted this way of getting CCW_DEVICE(&t->cdev) to the CcwDevice
form terminal_read (the pattern used at multiple places in the file). As
far as I can tell, the overhead basically depends on CONFIG_QOM_CAST_DEBUG.

I have no idea what do we have in production, but my guess is, that
ONFIG_QOM_CAST_DEBUG makes only sense for development and testing
(especially if proper test coverage is assumed).
Can you enlighten me?

CCW_DEVICE() may contain a run-time check (depending on CONFIG_QOM_CAST_DEBUG),
we however can make sure things are OK at compile time. This brings
me to the next question. Does it even make sense to use OBJECT_CHECK based
constructs when going from specific to general (we don't actually need a
cast here)? Obviously, for the other direction we really need a cast, so doing
a run-time check there does indeed provide added value.

Halil


>>      t->in_len -= len;
>>  
>>      return len;