Re: [RFC ABI V2 5/8] RDMA/core: Add new ioctl interface

[Christoph Hellwig <hch-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>]

On Wed, Jul 20, 2016 at 11:44:39PM -0600, Jason Gunthorpe wrote:
> How many ioctls have you used that use a complex variable sized struct
> as the parameter?
> 
> There are a few and they ones I've used are self-describing one way or
> another. Perhaps they have an op code or something in the struct, or a
> netlink-like format, or *something* but you don't need to know what
> the fd is connected to in order to parse the struct. The ioctl # and
> the argument should be enough to parse.

In that case they are designed by taste-less fools.  No need to
to spread that BS even further.

> > Well there is no need to do that with the standard way. The filehandle
> > identifies the driver. No need to modify strace and no need to
> > schlepp the device_id around.
> 
> Nope, that isn't how strace works, strace never checks the filehandle.

Strace is just a random tracing tool.  No need to design interface
for it.  Just use a ioctl values that don't confluct with anything
heavily used and we'll be fine.

> You are really surprised by the rdma_cm architecture??? I know it is
> goofy, but we are stuck with it..

Are we?  We'll need to stick to the wire format obviously, but the
software architectures needs to die rather sooner than later.  It's the
worst pile of junk in the RDMA subsystem, and that really means
something.  It's almost impossible to use it correctly due to all the
warts and un- or under-documented assumtions in it's APIs.

> .. and we have been doing the un-described structs like you are asking
> for today. libmlx4 just assumes the fd it is talking to is a mlx4
> driver (because sysfs said so) and jams in untagged driver-specific
> structs to the command flow. strace cannot parse them and there is no
> kernel support for debugging or accidental misuse..
> 
> Adding a little tag to the driver specific struct seems totally
> harmless, every iteration of the various proposals has had some kind
> of struct tag one way or another. I don't know what you see strange
> about this.

Just use different ioctl codes for different drivers and you're done.
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