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

From: James Smart <[email-masked]>
Date: Thu, 20 Apr 2006 10:33:12 -0400

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
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.

> ... I think there may be issues with packing structs or 32 bit
> userspace and 64 bit kernels and other fun things like this so the iscsi
> pdu and iscsi event have to be defined correctly and I guess we are back
> to some of the problems with ioctls :(

Agreed. In this use of netlink, there's not a lot of wins for netlink over
ioctls. It all comes down to 2 things: a) proper portable message definition;
and b) what do you do with that non-portable user space buffer pointer ?

-- james s