linux-kernel - Re: [PATCH 4/4][RFC] kvm: eoi

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Date:	Sun, 24 Jun 2012 18:40:50 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Alex Williamson <alex.williamson@...hat.com>
Cc:	avi@...hat.com, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	jan.kiszka@...mens.com
Subject: Re: [PATCH 4/4][RFC] kvm: eoi_eventfd

On Sun, Jun 24, 2012 at 08:47:57AM -0600, Alex Williamson wrote:
> On Sun, 2012-06-24 at 11:24 +0300, Michael S. Tsirkin wrote:
> > On Fri, Jun 22, 2012 at 04:16:53PM -0600, Alex Williamson wrote:
> > > I think we're probably also going to need something like this.
> > > When running in non-accelerated qemu, we're going to have to
> > > create some kind of EOI notifier for drivers.  VFIO can make
> > > additional improvements when running on KVM so it will probably
> > > make use of the KVM_IRQFD_LEVEL_EOI interface, but we don't
> > > want to have a generic EOI notifier in qemu that just stops
> > > working when kvm-ioapic is enabled.  This is just a simple way
> > > to register an eventfd using the existing KVM ack notifier.
> > > I tried combining the ack notifier of the LEVEL_EOI interface
> > > into this one, but it didn't work out well.  The code complexity
> > > goes up a lot.
> > > 
> > > Signed-off-by: Alex Williamson <alex.williamson@...hat.com>
> > > ---
> > > 
> > >  Documentation/virtual/kvm/api.txt |   14 ++++++
> > >  arch/x86/kvm/x86.c                |    1 
> > >  include/linux/kvm.h               |   12 +++++
> > >  include/linux/kvm_host.h          |    7 +++
> > >  virt/kvm/eventfd.c                |   89 +++++++++++++++++++++++++++++++++++++
> > >  virt/kvm/kvm_main.c               |    9 ++++
> > >  6 files changed, 132 insertions(+)
> > > 
> > > diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> > > index 2f8a0aa..69b1747 100644
> > > --- a/Documentation/virtual/kvm/api.txt
> > > +++ b/Documentation/virtual/kvm/api.txt
> > > @@ -1998,6 +1998,20 @@ matched using kvm_irqfd.fd, kvm_irqfd.gsi, and kvm_irqfd.fd2.
> > >  De-assigning automatically de-asserts the interrupt line setup through
> > >  this interface.
> > >  
> > > +4.77 KVM_EOI_EVENTFD
> > > +
> > > +Capability: KVM_CAP_EOI_EVENTFD
> > > +Architectures: x86
> > > +Type: vm ioctl
> > > +Parameters: struct kvm_eoi_eventfd (in)
> > > +Returns: 0 on success, -1 on error
> > > +
> > > +This interface allows userspace to be notified through an eventfd for
> > > +EOI writes to the in-kernel irqchip.  kvm_eoi_eventfd.fd specifies
> > > +the eventfd to signal on EOI to kvm_eoi_eventfd.gsi.  To disable,
> > > +use KVM_EOI_EVENTFD_FLAG_DEASSIGN and specify both the original fd
> > > +and gsi.
> > > +
> > >  5. The kvm_run structure
> > >  ------------------------
> > >  
> > 
> > The patch looks like it only works for ioapic IRQs.  I think it's a good
> > idea to explicitly limit this to level interrupts, straight away:
> > there's no reason for userspace to need an exit on an edge interrupt.
> 
> Are you suggesting documentation or code to prevent users from binding
> to an edge interrupt?

Yes.

> > I also suggest we put LEVEL somewhere in the name.
> 
> Ok
> 
> > With this eventfd, do we also need the fd2 parameter in the irqfd
> > structure? It seems to be used for the same thing ...
> 
> Yes we still need fd2, this does not replace KVM_IRQFD_LEVEL_EOI.  The
> models we need to support are:
> 
> 1) assert from userspace (IRQ_LINE), eoi to userspace (EOI_EVENTFD),
> de-assert from userspace (IRQ_LINE)
> 
> 2a) assert from vfio (IRQFD.fd), de-assert in kvm, notify vfio
> (IRQFD.fd2)
> or
> 
> 2b) assert from vfio (IRQFD.fd), eoi to vfio (EOI_EVENTFD), de-assert
> from vfio (IRQFD.fd2)

Or maybe deassert in kvm, notify vfio using eventfd?

> 
> This series enables 1) and 2a).  2b) has some pros and cons.  The pro is
> that vfio could test the device to see if an interrupt is pending and
> not de-assert if there is still an interrupt pending.  The con is that a
> standard level interrupt cycle takes 3 transactions instead of 2.
> 
> Any case of triggering the interrupt externally via an irqfd requires
> that the irqfd uses it's own irq_source_id so that it doesn't interfere
> with KVM_USERSPACE_IRQ_SOURCE_ID.  So we can't mix IRQ_LINE and IRQFD
> unless we want to completely expose a new irq source id allocation
> mechanism and include it in all the interfaces (KVM_GET_IRQ_SOURCE_ID,
> KVM_PUT_IRQ_SOURCE_ID, pass a source id to IRQFD and IRQ_LINE, etc).
> Thanks,
> 
> Alex

I don't follow this last paragraph. What are you trying to say?
To support IRQ sharing for level irqs, each irqfd for level IRQ
must have a distinct source id. But IRQ_LINE works fine without,
and one source ID for all irq_line users seems enough.

-- 
MST
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