[PATCH-RFC 0/2] eventfd: new EFD_STATE flag

["Michael S. Tsirkin" <mst@redhat.com>]

Davide, all,

This RFC series implements a new EFD_STATE flag for eventfd.
When set, this changes eventfd behaviour in the following way:
- write simply stores the value written, and is always non-blocking
- read unblocks when the value written changes, and
  returns the value written

Motivation: we'd like to use eventfd in qemu to pass interrupts from
(emulated or assigned) devices to guest. For level interrupts, the
counter supported currently by eventfd is not a good match: we really
need to set interrupt to a level, typically 0 or 1, wake the guest if
there was a change and and give the guest ability to see the last value
written.

Does extending eventfd in this way make sense?  Please comment.

Michael S. Tsirkin (2):
  eventfd: reorganize the code to simplify new flags
  eventfd: EFD_STATE flag

 fs/eventfd.c            |   83 +++++++++++++++++++++++++++++++++++++++--------
 include/linux/eventfd.h |    3 +-
 2 files changed, 71 insertions(+), 15 deletions(-)