Re: [PATCH 1/2] Allow returning EventNotifier's wfd

[Alex Williamson]

On Wed, 2 Mar 2022 16:23:42 +0100
Sergio Lopez <slp@redhat.com> wrote:

> On Wed, Mar 02, 2022 at 08:12:34AM -0700, Alex Williamson wrote:
> > On Wed,  2 Mar 2022 12:36:43 +0100
> > Sergio Lopez <slp@redhat.com> wrote:
> >   
> > > event_notifier_get_fd(const EventNotifier *e) always returns
> > > EventNotifier's read file descriptor (rfd). This is not a problem when
> > > the EventNotifier is backed by a an eventfd, as a single file
> > > descriptor is used both for reading and triggering events (rfd ==
> > > wfd).
> > > 
> > > But, when EventNotifier is backed by a pipefd, we have two file
> > > descriptors, one that can only be used for reads (rfd), and the other
> > > only for writes (wfd).
> > > 
> > > There's, at least, one known situation in which we need to obtain wfd
> > > instead of rfd, which is when setting up the file that's going to be
> > > sent to the peer in vhost's SET_VRING_CALL.
> > > 
> > > Extend event_notifier_get_fd() to receive an argument which indicates
> > > whether the caller wants to obtain rfd (false) or wfd (true).  
> > 
> > There are about 50 places where we add the false arg here and 1 where
> > we use true.  Seems it would save a lot of churn to hide this
> > internally, event_notifier_get_fd() returns an rfd, a new
> > event_notifier_get_wfd() returns the wfd.  Thanks,  
> 
> I agree. In fact, that's what I implemented in the first place. I
> changed to this version in which event_notifier_get_fd() is extended
> because it feels more "correct". But yes, the pragmatic option would
> be adding a new event_notifier_get_wfd().
> 
> I'll wait for more reviews, and unless someone voices against it, I'll
> respin the patches with that strategy (I already have it around here).

I'd argue that adding a bool as an arg to a function to change the
return value is sufficiently non-intuitive to program for that the
wrapper method is actually more correct.  event_notifier_get_fd()
essentially becomes a shorthand for event_notifier_get_rfd().  Thanks,

Alex