Re: Re: another change breaks rxe

[Jason Gunthorpe <jgg@nvidia.com>]

On Fri, Oct 30, 2020 at 12:50:01PM +0000, Bernard Metzler wrote:

> I see the hfi and ipath drivers similarly using ibv_cmd_post_send
> to push the wqe's (while not having them in shared mem). Do they
> brake as well?
> Do we have an example of decent eventfd usage where I could learn
> from how to use it?

You'd add a uAPI accepting a eventfd and then call a sequence like:

        init_waitqueue_func_entry(.., my_func);
        init_poll_funcptr(..);

        events = vfs_poll(evfile, ..);

And my_func will be called when the user does write() on the event fd.

drivers/vfio/virqfd.c has the simplest version of this

It could be some core code to do this setup with a new driver callback
'user_doorbell_trigger' or something like that.

However, I see at closer inspection that rxe and siw both want the QPN
to be passed in, so this doesn't look like it would work - an eventfd
per QP is too much overhead.

But calling post_send, even with a 0 length, is really slow.

Jason