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

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

On Thu, Jul 19, 2012 at 02:12:13PM +0300, Michael S. Tsirkin wrote:
> On Thu, Jul 19, 2012 at 01:54:53PM +0300, Gleb Natapov wrote:
> > On Thu, Jul 19, 2012 at 01:26:48PM +0300, Michael S. Tsirkin wrote:
> > > 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.
> > > 
> > So this is again eventfd and level interrupts incompatibility problem?
> 
> At some level, yes.
> 
> > > > 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.
> > It returns 0 if interrupt was coalesced. It was not.
> 
> Not really, if you call it with level 0 you always get 1 back.
> Look at kvm_ioapic_set_irq, see what happens if level is 0.
> It looks like a bug though a harmless one.
> 
> > > 
> > > > > 
> > > > > 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.
> > > 
> > Can you point me to exact problem in Alex's patch?
> 
> It's very simple. Alex adds an interface to clear the level
> automatically from guest on EOI.  So the caller has no way to know the
> current state for a given source ID and can not restore it after
> migration.

That's one problem :). The second problem is it is adding more spinlocks
around kvm_set_irq, so if we want to avoid vcpu scans under spinlock,
we'll have more work to do. I'm not sure how serious this second
problem is, Avi nacked a patch because of a similar issue in the past
but that had to deal with MSI.

> > --
> > 			Gleb.