Re: [PATCH v5 1/4] kvm: Extend irqfd to support level interrupts

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

On Wed, Jul 18, 2012 at 01:55:30PM +0300, Gleb Natapov wrote:
> On Wed, Jul 18, 2012 at 01:53:11PM +0300, Michael S. Tsirkin wrote:
> > On Wed, Jul 18, 2012 at 01:49:06PM +0300, Gleb Natapov wrote:
> > > On Wed, Jul 18, 2012 at 01:48:44PM +0300, Michael S. Tsirkin wrote:
> > > > On Wed, Jul 18, 2012 at 01:44:29PM +0300, Gleb Natapov wrote:
> > > > > On Wed, Jul 18, 2012 at 01:41:14PM +0300, Michael S. Tsirkin wrote:
> > > > > > On Mon, Jul 16, 2012 at 02:33:47PM -0600, Alex Williamson wrote:
> > > > > > > In order to inject a level interrupt from an external source using an
> > > > > > > irqfd, we need to allocate a new irq_source_id.  This allows us to
> > > > > > > assert and (later) de-assert an interrupt line independently from
> > > > > > > users of KVM_IRQ_LINE and avoid lost interrupts.
> > > > > > > 
> > > > > > > We also add what may appear like a bit of excessive infrastructure
> > > > > > > around an object for storing this irq_source_id.  However, notice
> > > > > > > that we only provide a way to assert the interrupt here.  A follow-on
> > > > > > > interface will make use of the same irq_source_id to allow de-assert.
> > > > > > > 
> > > > > > > Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> > > > > > > ---
> > > > > > > 
> > > > > > >  Documentation/virtual/kvm/api.txt |    6 ++
> > > > > > >  arch/x86/kvm/x86.c                |    1 
> > > > > > >  include/linux/kvm.h               |    3 +
> > > > > > >  virt/kvm/eventfd.c                |  114 ++++++++++++++++++++++++++++++++++++-
> > > > > > >  4 files changed, 120 insertions(+), 4 deletions(-)
> > > > > > > 
> > > > > > > diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> > > > > > > index 100acde..c7267d5 100644
> > > > > > > --- a/Documentation/virtual/kvm/api.txt
> > > > > > > +++ b/Documentation/virtual/kvm/api.txt
> > > > > > > @@ -1981,6 +1981,12 @@ the guest using the specified gsi pin.  The irqfd is removed using
> > > > > > >  the KVM_IRQFD_FLAG_DEASSIGN flag, specifying both kvm_irqfd.fd
> > > > > > >  and kvm_irqfd.gsi.
> > > > > > >  
> > > > > > > +The KVM_IRQFD_FLAG_LEVEL flag indicates the gsi input is for a level
> > > > > > > +triggered interrupt.  In this case a new irqchip input is allocated
> > > > > > > +which is logically OR'd with other inputs allowing multiple sources
> > > > > > > +to independently assert level interrupts.  The KVM_IRQFD_FLAG_LEVEL
> > > > > > > +is only necessary on setup, teardown is identical to that above.
> > > > > > > +KVM_IRQFD_FLAG_LEVEL support is indicated by KVM_CAP_IRQFD_LEVEL.
> > > > > > >  
> > > > > > >  5. The kvm_run structure
> > > > > > >  ------------------------
> > > > > > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > > > > > index a01a424..80bed07 100644
> > > > > > > --- a/arch/x86/kvm/x86.c
> > > > > > > +++ b/arch/x86/kvm/x86.c
> > > > > > > @@ -2148,6 +2148,7 @@ int kvm_dev_ioctl_check_extension(long ext)
> > > > > > >  	case KVM_CAP_GET_TSC_KHZ:
> > > > > > >  	case KVM_CAP_PCI_2_3:
> > > > > > >  	case KVM_CAP_KVMCLOCK_CTRL:
> > > > > > > +	case KVM_CAP_IRQFD_LEVEL:
> > > > > > >  		r = 1;
> > > > > > >  		break;
> > > > > > >  	case KVM_CAP_COALESCED_MMIO:
> > > > > > > diff --git a/include/linux/kvm.h b/include/linux/kvm.h
> > > > > > > index 2ce09aa..b2e6e4f 100644
> > > > > > > --- a/include/linux/kvm.h
> > > > > > > +++ b/include/linux/kvm.h
> > > > > > > @@ -618,6 +618,7 @@ struct kvm_ppc_smmu_info {
> > > > > > >  #define KVM_CAP_PPC_GET_SMMU_INFO 78
> > > > > > >  #define KVM_CAP_S390_COW 79
> > > > > > >  #define KVM_CAP_PPC_ALLOC_HTAB 80
> > > > > > > +#define KVM_CAP_IRQFD_LEVEL 81
> > > > > > >  
> > > > > > >  #ifdef KVM_CAP_IRQ_ROUTING
> > > > > > >  
> > > > > > > @@ -683,6 +684,8 @@ struct kvm_xen_hvm_config {
> > > > > > >  #endif
> > > > > > >  
> > > > > > >  #define KVM_IRQFD_FLAG_DEASSIGN (1 << 0)
> > > > > > > +/* Available with KVM_CAP_IRQFD_LEVEL */
> > > > > > > +#define KVM_IRQFD_FLAG_LEVEL (1 << 1)
> > > > > > >  
> > > > > > >  struct kvm_irqfd {
> > > > > > >  	__u32 fd;
> > > > > > > diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
> > > > > > > index 7d7e2aa..ecdbfea 100644
> > > > > > > --- a/virt/kvm/eventfd.c
> > > > > > > +++ b/virt/kvm/eventfd.c
> > > > > > > @@ -36,6 +36,68 @@
> > > > > > >  #include "iodev.h"
> > > > > > >  
> > > > > > >  /*
> > > > > > > + * An irq_source_id can be created from KVM_IRQFD for level interrupt
> > > > > > > + * injections and shared with other interfaces for EOI or de-assert.
> > > > > > > + * Create an object with reference counting to make it easy to use.
> > > > > > > + */
> > > > > > > +struct _irq_source {
> > > > > > > +	int id; /* the IRQ source ID */
> > > > > > > +	bool level_asserted; /* Track assertion state and protect with lock */
> > > > > > > +	spinlock_t lock;     /* to avoid unnecessary re-assert/spurious eoi. */
> > > > > > > +	struct kvm *kvm;
> > > > > > > +	struct kref kref;
> > > > > > > +};
> > > > > > > +
> > > > > > > +static void _irq_source_release(struct kref *kref)
> > > > > > > +{
> > > > > > > +	struct _irq_source *source;
> > > > > > > +
> > > > > > > +	source = container_of(kref, struct _irq_source, kref);
> > > > > > > +
> > > > > > > +	/* This also de-asserts */
> > > > > > > +	kvm_free_irq_source_id(source->kvm, source->id);
> > > > > > > +	kfree(source);
> > > > > > > +}
> > > > > > > +
> > > > > > > +static void _irq_source_put(struct _irq_source *source)
> > > > > > > +{
> > > > > > > +	if (source)
> > > > > > > +		kref_put(&source->kref, _irq_source_release);
> > > > > > > +}
> > > > > > > +
> > > > > > > +static struct _irq_source *__attribute__ ((used)) /* white lie for now */
> > > > > > > +_irq_source_get(struct _irq_source *source)
> > > > > > > +{
> > > > > > > +	if (source)
> > > > > > > +		kref_get(&source->kref);
> > > > > > > +
> > > > > > > +	return source;
> > > > > > > +}
> > > > > > > +
> > > > > > > +static struct _irq_source *_irq_source_alloc(struct kvm *kvm)
> > > > > > > +{
> > > > > > > +	struct _irq_source *source;
> > > > > > > +	int id;
> > > > > > > +
> > > > > > > +	source = kzalloc(sizeof(*source), GFP_KERNEL);
> > > > > > > +	if (!source)
> > > > > > > +		return ERR_PTR(-ENOMEM);
> > > > > > > +
> > > > > > > +	id = kvm_request_irq_source_id(kvm);
> > > > > > > +	if (id < 0) {
> > > > > > > +		kfree(source);
> > > > > > > +		return ERR_PTR(id);
> > > > > > > +	}
> > > > > > > +
> > > > > > > +	kref_init(&source->kref);
> > > > > > > +	spin_lock_init(&source->lock);
> > > > > > > +	source->kvm = kvm;
> > > > > > > +	source->id = id;
> > > > > > > +
> > > > > > > +	return source;
> > > > > > > +}
> > > > > > > +
> > > > > > > +/*
> > > > > > >   * --------------------------------------------------------------------
> > > > > > >   * irqfd: Allows an fd to be used to inject an interrupt to the guest
> > > > > > >   *
> > > > > > > @@ -52,6 +114,8 @@ struct _irqfd {
> > > > > > >  	/* Used for level IRQ fast-path */
> > > > > > >  	int gsi;
> > > > > > >  	struct work_struct inject;
> > > > > > > +	/* IRQ source ID for level triggered irqfds */
> > > > > > > +	struct _irq_source *source;
> > > > > > >  	/* Used for setup/shutdown */
> > > > > > >  	struct eventfd_ctx *eventfd;
> > > > > > >  	struct list_head list;
> > > > > > > @@ -62,7 +126,7 @@ struct _irqfd {
> > > > > > >  static struct workqueue_struct *irqfd_cleanup_wq;
> > > > > > >  
> > > > > > >  static void
> > > > > > > -irqfd_inject(struct work_struct *work)
> > > > > > > +irqfd_inject_edge(struct work_struct *work)
> > > > > > >  {
> > > > > > >  	struct _irqfd *irqfd = container_of(work, struct _irqfd, inject);
> > > > > > >  	struct kvm *kvm = irqfd->kvm;
> > > > > > > @@ -71,6 +135,29 @@ irqfd_inject(struct work_struct *work)
> > > > > > >  	kvm_set_irq(kvm, KVM_USERSPACE_IRQ_SOURCE_ID, irqfd->gsi, 0);
> > > > > > >  }
> > > > > > >  
> > > > > > > +static void
> > > > > > > +irqfd_inject_level(struct work_struct *work)
> > > > > > > +{
> > > > > > > +	struct _irqfd *irqfd = container_of(work, struct _irqfd, inject);
> > > > > > > +
> > > > > > > +	/*
> > > > > > > +	 * Inject an interrupt only if not already asserted.
> > > > > > > +	 *
> > > > > > > +	 * We can safely ignore the kvm_set_irq return value here.  If
> > > > > > > +	 * masked, the irr bit is still set and will eventually be serviced.
> > > > > > > +	 * This interface does not guarantee immediate injection.  If
> > > > > > > +	 * coalesced, an eoi will be coming where we can de-assert and
> > > > > > > +	 * re-inject if necessary.  NB, if you need to know if an interrupt
> > > > > > > +	 * was coalesced, this interface is not for you.
> > > > > > > +	 */
> > > > > > > +	spin_lock(&irqfd->source->lock);
> > > > > > > +	if (!irqfd->source->level_asserted) {
> > > > > > > +		kvm_set_irq(irqfd->kvm, irqfd->source->id, irqfd->gsi, 1);
> > > > > > > +		irqfd->source->level_asserted = true;
> > > > > > > +	}
> > > > > > > +	spin_unlock(&irqfd->source->lock);
> > > > > > > +}
> > > > > > > +
> > > > > > 
> > > > > > So as was discussed kvm_set_irq under spinlock is bad for scalability
> > > > > > with multiple VCPUs.  Why do we need a spinlock simply to protect
> > > > > > level_asserted?  Let's use an atomic test and set/test and clear and the
> > > > > > problem goes away.
> > > > > > 
> > > > > That sad reality is that for level interrupt we already scan all vcpus
> > > > > under spinlock.
> > > > 
> > > > Where?
> > > > 
> > > ioapic
> > 
> > $ grep kvm_for_each_vcpu virt/kvm/ioapic.c
> > $
> > 
> > ?
> > 
> 
> Come on Michael. You can do better than grep and actually look at what
> code does. The code that loops over all vcpus while delivering an irq is
> in kvm_irq_delivery_to_apic(). Now grep for that.

Hmm, I see, it's actually done for edge if injected from ioapic too,
right?

So set_irq does a linear scan, and for each matching CPU it calls
kvm_irq_delivery_to_apic which is another scan?
So it's actually N^2 worst case for a broadcast?

> --
> 			Gleb.