linux-kernel - Re: [PATCH v7 2/2] kvm: KVM

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Message-ID: <20120812075324.GB1421@redhat.com>
Date:	Sun, 12 Aug 2012 10:53:24 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	Alex Williamson <alex.williamson@...hat.com>, gleb@...hat.com,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	jan.kiszka@...mens.com
Subject: Re: [PATCH v7 2/2] kvm: KVM_EOIFD, an eventfd for EOIs

On Mon, Aug 06, 2012 at 01:17:12PM +0300, Avi Kivity wrote:
> On 07/24/2012 11:43 PM, Alex Williamson wrote:
> > This new ioctl enables an eventfd to be triggered when an EOI is
> > written for a specified irqchip pin.  The first user of this will
> > be external device assignment through VFIO, using a level irqfd
> > for asserting a PCI INTx interrupt and this interface for de-assert
> > and notification once the interrupt is serviced.
> > 
> > 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.
> 
> The name is slightly misleading.  eoifd doesn't trigger on EOI (which is
> an APIC->IOAPIC interface) but rather when an interrupt controller
> resamples an input line.  This happens for the IOAPIC when an EOI is
> received for a vector that is configured for level interrupts and not
> masked, or similarly for a PIC (but this is not triggered by an APIC EOI).
> 
> It's not a huge difference, but let's document it.

In fact, when we really need to notify userspace is after
we auto-deassert an interrupt: userspace does not
need an EOI notification as such.


> >  
> > +4.77 KVM_EOIFD
> > +
> > +Capability: KVM_CAP_EOIFD
> > +Architectures: x86
> > +Type: vm ioctl
> > +Parameters: struct kvm_eoifd (in)
> > +Returns: 0 on success, < 0 on error
> > +
> > +KVM_EOIFD allows userspace to receive interrupt EOI notification
> > +through an eventfd.  kvm_eoifd.fd specifies the eventfd used for
> > +notification.  KVM_EOIFD_FLAG_DEASSIGN is used to de-assign an eoifd
> > +once assigned.  KVM_EOIFD also requires additional bits set in
> > +kvm_eoifd.flags to bind to the proper interrupt line.  The
> > +KVM_EOIFD_FLAG_LEVEL_IRQFD indicates that kvm_eoifd.key is provided
> > +and is a key from a level triggered interrupt (configured from
> > +KVM_IRQFD using KVM_IRQFD_FLAG_LEVEL).  The EOI notification is bound
> > +to the same GSI and irqchip input as the irqfd.  Both kvm_eoifd.key
> > +and KVM_EOIFD_FLAG_LEVEL_IRQFD must be specified on assignment and
> > +de-assignment of KVM_EOIFD.  A level irqfd may only be bound to a
> > +single eoifd.  KVM_CAP_EOIFD_LEVEL_IRQFD indicates support of
> > +KVM_EOIFD_FLAG_LEVEL_IRQFD.
> 
> Why do we need to couple eoifd and irqfd?
> 
> 
> 
> -- 
> error compiling committee.c: too many arguments to function
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