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

On Wed, Jul 20, 2016 at 07:57:09PM -0500, Christoph Lameter wrote:

quoted

The device_id part of the extension allows each driver to have its own
globally unique number space without requireing another ioctl block.

But you already have that uniqueness if the ioctl filehandle provides
that.

The argument is that ioctls should be self-describing and never rely
on the filehandle to uniq them. That is basically standard in the
kernel, and why we have Documentation/ioctl/ioctl-number.txt
describing how to uniquely assign ioctl numbers.

There is nothing special here, we are just creating a much larger
ioctl # space by using (ioctl #, operation, device_id) as the globally
unique key, following existing best-practice if self-identifying ioctls.

multi-device requirements in the network code require a new device that
aggregates the other ones. The aggregation device is not world accessible.
The same approach could be used for RDMA.

Some do, some don't. The rdma_cm requirement is more like
listen(0.0.0.0) which does not require special net device aggregation.

quoted

No, strace doesn't know what an open FD is, it just inspects the ioctl
# and content to do its decode. Requiring strace to decode differently
depending on what was opened would be the deviation from what we do
today.

It is the  basic Unix convention to have a descriptor/file handle refer to
a device. There is nothing special required here from strace.

Sometimes yes, sometimes no. There are lots of examples both ways.

/dev/pts/ptmx, /dev/mapper/control, /dev/btrfs-control, etc, etc are
all examples of multi-device control fds similar to the proposed
single char dev.

.. and bear in mind that /dev/uverbs0 doesn't even really make that
much sense as it aggregates two physical ports. There is already no
way to split permissions up by port, which is a logical thing to want
for some dual-rail configurations.

What we have today really doesn't make much sense.

Jason
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