On Mon, 22 Jun 2009, Gregory Haskins wrote: > The general thesis is for decoupling of the two subsystems. In order to > do this, you need some form of polymorphism and an intermediate "handle" > mechanism which is userspace friendly. File-descriptors already fit > this role neatly, with the "int fd" being the handle, and the f_ops > being the polymorphic interface. Eventfd is of course, a subclass of > this concept in that it has these same general properties but with > signaling semantics (non-blocking collapsible events, etc). > > Say, for example, you wanted disk IO completion events to generate an > interrupt into a guest. One way to do this would, of course, modify all > the disk-io code so it knows how to directly inject a KVM guest > interrupt. While this would work, someone would undoubtedly get flamed > for such a suggestion ;) > > Another way to do it is to treat the AIO eventfd as the hook point. > IIUC AIO already knows how to be an eventfd producer. KVM, by virtue of > irqfd, already knows how to be an eventfd consumer. So now kvm can > consume AIO, or it can consume userspace events equally well, and > without modification. Neither side needs to know about the other per > se, other than the details on how to use the eventfd interface. > > Don't get me wrong: We expect userspace to use all this stuff too. I > just expect that we will see all permutations of producer/consumer + > userspace/kernel combinations, so I want to retain that "all producers > have left" notification feature set. Today eventfd supports producers > or consumers in userspace, and producers in the kernel. This new work > we are doing adds consumer support in the kernel. Kernel to kernel is > just a natural extension of that. A file* is the VFS link between userspace and the kernel. Is not a magical polymorphic interface to be used for whatever kernel side reasons. Basing a kernel internal API over it is flawed. On top of that, a single reference count does not put you on cover about the possible combinations of producers and consumers. For that, you'd need a pipe-like reference handling logic, that is way far from the eventfd scope. So please stop making hypothetical cases about interface usages, and *when* we will have a real case, we'll see what the better handling for it will be. - Davide -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
