Re: Ummunotify: progress at last!

[Roland Dreier <rdreier-FYB4Gu1CFyUAvxtiuMwx3w@public.gmane.org>]

 > No, there is no mmap. Like this:
 > 
 > u64 my_counter = 0;
 > 
 > ibv_set_mmu_counter(verbs, &my_counter);
 > [..]
 > while (my_counter != last_my_counter) {
 >     last_my_counter = my_counter;
 >     ibv_get_mmu_notifications(verbs, ...);   // <- I am a memory barrier as well
 > }
 > 
 > The kernel 'syscall' ibv_set_mmu_counter would bind the given verbs to
 > the 8 byte counter you specified without having to the mmap thing. As
 > I understand it this is what perfevents does.
 > 
 > Integrating with the verbs api avoids the need for another device
 > file. That is good. Eliminating poll() from the API can remove the
 > dedicated fd entirely. Within verbs I guess we could replace poll()
 > with something like a completion channel (??) if anyone cares.

That is all definitely doable.  I wonder if it's better to get rid of
the dedicated fd though.  After all, having the fd means a fancy app can
do poll() or sigio or whatever internally.  Being able to integrate into
an fd-driven event loop seems like a pretty big thing to give up.

Also, having a uverbs "syscall" that is exactly like read() seems a bit
of a stretch, even within the ugliness that is the uverbs interface.
(We love all our children, but sometimes we have to be realistic)

 - R.
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Roland Dreier  <rolandd-FYB4Gu1CFyUAvxtiuMwx3w@public.gmane.org>
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