Re: [RFC] Netlink and user-space buffer pointers

[Mike Christie <michaelc@cs.wisc.edu>]

Mike Christie wrote:
> James Smart wrote:
>> Mike Christie wrote:
>>> For the tasks you want to do for the fc class is performance critical?
>> No, it should not be.
>>
>>> If not, you could do what the iscsi class (for the netdev people this is
>>> drivers/scsi/scsi_transport_iscsi.c) does and just suffer a couple
>>> copies. For iscsi we do this in userspace to send down a login pdu:
>>>
>>>     /*
>>>      * xmitbuf is a buffer that is large enough for the iscsi_event,
>>>      * iscsi pdu (hdr_size) and iscsi pdu data (data_size)
>>>      */
>> Well, the real difference is that the payload of the "message" is actually
>> the payload of the SCSI command or ELS/CT Request. Thus, the payload may
> 
> I am not sure I follow. For iscsi, everything after the iscsi_event
> struct can be the iscsi request that is to be transmitted. The payload
> will not normally be Mbytes but it is not a couple if bytes.
> 
>> range in size from a few hundred bytes to several kbytes (> 1 page) to
>> Mbyte's in size. Rather than buffer all of this, and push it over the
>> socket,
>> thus the extra copies - it would best to have the LLDD simply DMA the
>> payload like on a typical SCSI command.  Additionally, there will be
>> response data that can be several kbytes in length.
>>
> 
> Once you have got the buffer to the class, the class can create a
> scatterlist to DMA from for the LLD. I thought. iscsi does not do this
> just because it is software right now. For qla4xxx we do not need

That should be, we do need.