Re: [PATCH v8 0/6] kvm: level irqfd support

[Alex Williamson <alex.williamson@redhat.com>]

On Wed, 2012-08-15 at 22:22 +0300, Michael S. Tsirkin wrote:
> On Wed, Aug 15, 2012 at 11:36:31AM -0600, Alex Williamson wrote:
> > On Wed, 2012-08-15 at 17:28 +0300, Michael S. Tsirkin wrote:
> > > On Fri, Aug 10, 2012 at 04:37:08PM -0600, Alex Williamson wrote:
> > > > v8:
> > > > 
> > > > Trying a new approach.  Nobody seems to like the internal IRQ
> > > > source ID object and the interactions it implies between irqfd
> > > > and eoifd, so let's get rid of it.  Instead, simply expose
> > > > IRQ source IDs to userspace.  This lets the user be in charge
> > > > of freeing them or hanging onto a source ID for later use.
> > > 
> > > In the end it turns out source ID is an optimization for shared
> > > interrupts, isn't it?  Can't we apply the optimization transparently to
> > > the user?  E.g. if we have some spare source IDs, allocate them, if we
> > > run out, use a shared source ID?
> > 
> > Let's think about shared source IDs a bit more.  I think it's wrong that
> > irqfd uses KVM_USERSPACE_IRQ_SOURCE_ID, but I'm questioning whether all
> > irqfd users can share a source ID.  We do not get the logical OR of all
> > users by putting them on the same source ID, we get "last set wins".
> > KVM_USERSPACE_IRQ_SOURCE_ID is used for multiple inputs because the
> > logical OR happens in userspace.  How would we not starve a user if we
> > define KVM_IRQFD_SOURCE_ID?  What am I missing?
> 
> That all irqfds are deasserted on EOI anyway.  So there's no point
> to do a logical OR.

Ok, so the argument is:

- edge irqfds (the code now) can share a source ID because there is no
state.  Overlapping interrupt injects always cause one or more edge
triggers.
- your proposed level extension can only be asserted by the inject
eventfd and is only de-asserted by EOI, which de-asserts and notifies
all users.

What prevents an edge irqfd being registered to the same GSI as a level
irqfd, resulting in a de-assert that might result in the irr not being
seen by the guest and therefore maybe not getting an EOI? (I think this
is the same problem as why we can't use the exiting irqfd to insert a
level interrupt)

Having the de-assert only on EOI policy allows level irqfds to share a
source ID, but do they all need to share a separate source ID from edge
irqfds?

> > So I'm inclined to say source IDs are a requirement for shared
> > interrupts.
> 
> Can yo show a specific example that breaks?
> I don't think it can exist.

Only the edge vs level interaction if we define the policy above for
de-assert.

> > That means the re-use scheme becomes complicated (ex. we
> > run out of IRQ source IDs, so we start looking for sharing by re-using a
> > source ID used by a different GSI).  Do we want to do that in kernel or
> > userspace?  This series allows userspace to deal with that complexity.
> > Please let me know if I'm thinking incorrectly about source ID re-use.
> > Thanks,
> > 
> > Alex
> 
> I think there is a misunderstanding.
> All deassert on ack irqfds can share a source ID.
> This is why I am now thinking deassert on ack behaviour
> should be set when irqfd is assigned.

Maybe you were already thinking along the lines of a separate source ID
for de-assert on ack irqfds vs normal irqfds then.  I think I missed
that.  Thanks,

Alex