Re: [KVM-RFC PATCH 0/2] irqfd: use POLLHUP notification for close()

Michael S. Tsirkin
2009-06-02T16:21:15Z

On Tue, Jun 02, 2009 at 12:14:15PM -0400, Gregory Haskins wrote:
> Michael S. Tsirkin wrote:
> > On Tue, Jun 02, 2009 at 11:15:28AM -0400, Gregory Haskins wrote:
> >
> >> (Applies to kvm.git/master:25deed73)
> >>
> >> Please see the header for 2/2 for a description. This patch series has
> >> been fully tested and appears to be working correctly. I have it as an RFC
> >> for now because it needs Davide's official submission/SOB for patch 1/2, and
> >> it should get some eyeballs/acks on my SRCU usage before going in.
> >>
> >> I will submit the updated irqfd userspace which eschews the deassign() verb
> >> since we can now just use the close(fd) method alone. I will also address
> >> the userspace review comments from Avi.
> >>
> >
> >
> > We are not killing the deassign though, do we?
> >
>
> Yes, it is not needed any more now that we have proper
> release-notification from eventfd.
>
> > It's good to have that option e.g. for when we pass
> > the fd to another process.
> >
>
> Passing the fd to another app should up the underlying file reference
> count. If the producer app wants to "deassign" it simply calls
> close(fd) (as opposed to today where it calls DEASSIGN+close), but the
> reference count will allow the consuming app to leave the eventfd's file
> open. Or am I misunderstanding you?
>
> -Greg
>
>

I think we want to keep supporting the deassign ioctl. This, even though
close overlaps with it functionally somewhat.

This allows qemu to pass eventfd to another process/device, and then
block/unblock interrupts as seen by that process by
assigning/deassigning irq to it. This is much easier and lightweight
than asking another process to close the fd and passing another fd
later.

--
MST