Re: [PATCH RFC untested] kvm_set_irq: report coalesced for clear

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

On Thu, Jul 19, 2012 at 12:41:24PM +0300, Gleb Natapov wrote:
> On Thu, Jul 19, 2012 at 12:33:29PM +0300, Michael S. Tsirkin wrote:
> > On Thu, Jul 19, 2012 at 12:21:07PM +0300, Gleb Natapov wrote:
> > > On Thu, Jul 19, 2012 at 12:17:19PM +0300, Michael S. Tsirkin wrote:
> > > > On Thu, Jul 19, 2012 at 10:53:37AM +0300, Gleb Natapov wrote:
> > > > > On Thu, Jul 19, 2012 at 01:11:53AM +0300, Michael S. Tsirkin wrote:
> > > > > > This creates a way to detect when kvm_set_irq(...,0) was run
> > > > > > twice with the same source id by returning 0 in this case.
> > > > > > 
> > > > > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > > > > > ---
> > > > > > 
> > > > > > This is on top of my bugfix patch.  Uncompiled and untested.  Alex, I
> > > > > > think something like this patch will make it possible for you to simply
> > > > > > do
> > > > > > 	if (kvm_set_irq(...., 0))
> > > > > > 		eventfd_signal()
> > > > > > 
> > > > > Why caller can't track line state?
> > > > 
> > > > Why duplicate information? As we are finding it's not trivial to keep
> > > > the two in sync. Think about migration etc ...
> > > > 
> > > We do not migrate irq_states. The caller already have to have enough
> > > information to recreate its state and it should migrate the info, so why
> > > should we go all the way down the call chain to find something that is
> > > already known?
> > 
> > Hmm it's an interesting point. Looks like irqfds for level lose state
> > across migration. Of course Alex wants to use them for assignment which
> > currently disables migration, but we are talking about a generic API,
> > so it's a problem that there's no way to retrieve the state.
> > 
> There is no any problem. Source knows what the line status is.

With EOIFD and level IRQFD, it does not.

> Furthermore this is a (benign) bug if device calls irq_set with
> the same level since it results in needless system calls. Qemu guilty
> of it and _that_ should be fixed.

Fine but we are arguably returning a wrong result in that case:
set_irq twice to 0 return 1 each time. I would expect 0 the
second time.

> > 
> > Also migration is only one example. Duplicated state is generally
> > nasty.  We would need extra locking too which is not nice.
> > 
> I don't know what extra locking you are talking about, but calling
> kvm_set_irq() repeatedly with the same level will do a lot of unnecessary
> locking in ioapic.

I am talking about Alex's EOIFD. This is what this patch is trying
to help.


> --
> 			Gleb.