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SubjectRe: [PATCH v2 5/6] kvm: KVM_EOIFD, an eventfd for EOIs
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On Wed, 2012-06-27 at 17:51 +0300, Gleb Natapov wrote:
> On Wed, Jun 27, 2012 at 08:29:04AM -0600, Alex Williamson wrote:
> > On Wed, 2012-06-27 at 16:58 +0300, Gleb Natapov wrote:
> > > On Tue, Jun 26, 2012 at 11:10:08PM -0600, Alex Williamson wrote:
> > > > This new ioctl enables an eventfd to be triggered when an EOI is
> > > > written for a specified irqchip pin. By default this is a simple
> > > > notification, but we can also tie the eoifd to a level irqfd, which
> > > > enables the irqchip pin to be automatically de-asserted on EOI.
> > > > This mode is particularly useful for device-assignment applications
> > > > where the unmask and notify triggers a hardware unmask. The default
> > > > mode is most applicable to simple notify with no side-effects for
> > > > userspace usage, such as Qemu.
> > > >
> > > > Here we make use of the reference counting of the _irq_source
> > > > object allowing us to share it with an irqfd and cleanup regardless
> > > > of the release order.
> > > >
> > > > Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> > > > ---
> > > >
> > > > Documentation/virtual/kvm/api.txt | 24 +++++
> > > > arch/x86/kvm/x86.c | 1
> > > > include/linux/kvm.h | 14 +++
> > > > include/linux/kvm_host.h | 13 +++
> > > > virt/kvm/eventfd.c | 189 +++++++++++++++++++++++++++++++++++++
> > > > virt/kvm/kvm_main.c | 11 ++
> > > > 6 files changed, 250 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> > > > index b216709..87a2558 100644
> > > > --- a/Documentation/virtual/kvm/api.txt
> > > > +++ b/Documentation/virtual/kvm/api.txt
> > > > @@ -1987,6 +1987,30 @@ interrupts with those injected through KVM_IRQ_LINE. IRQFDs created
> > > > with KVM_IRQFD_FLAG_LEVEL must also set this flag when de-assiging.
> > > > KVM_IRQFD_FLAG_LEVEL support is indicated by KVM_CAP_IRQFD_LEVEL.
> > > >
> > > > +4.77 KVM_EOIFD
> > > > +
> > > > +Capability: KVM_CAP_EOIFD
> > > > +Architectures: x86
> > > > +Type: vm ioctl
> > > > +Parameters: struct kvm_eoifd (in)
> > > > +Returns: 0 on success, -1 on error
> > > > +
> > > > +KVM_EOIFD allows userspace to receive EOI notification through an
> > > > +eventfd for level triggered irqchip interrupts. Behavior for edge
> > > > +triggered interrupts is undefined. kvm_eoifd.fd specifies the eventfd
> > > Lets make it defined. EOI notification can be used by userspace to fix
> > > time drift due to lost interrupts. But than userspace needs to know
> > > which vcpu did EOI.
> >
> > Hmm, do we need an additional flag and field in kvm_eoifd to filter by
> > vCPU then?
> >
> This will be enough for a use case I am aware of. Don't know if this
> interface is generic enough for all possible use cases.

That's generally a hard prediction to make ;) We currently don't pass a
kvm_vcpu anywhere close to the irq ack notifier. The ioapic path could
be relatively trivial, but the pic path is a bit further disconnected.
If we had that plumbing, a KVM_CAP plus vcpu filter flag and specifying
the vcpu using some of the padding space seems like it's sufficient.
I'll drop mention of level-only from the description, but the plumbing
and vcpu filtering can be a follow-on. Thanks,

Alex



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