Re: [PATCH v9 1/2] kvm: Use a reserved IRQ source ID for irqfd

Alex Williamson
2012-08-21T20:06:23Z

On Tue, 2012-08-21 at 22:58 +0300, Michael S. Tsirkin wrote:
> On Tue, Aug 21, 2012 at 01:29:06PM -0600, Alex Williamson wrote:
> > KVM_IRQFD currently uses the reserved KVM_USERSPACE_IRQ_SOURCE_ID
> > which is also shared with userspace injection methods like
> > KVM_IRQ_LINE. This can cause a conflict if an irqfd triggers on
> > a GSI asserted through KVM_IRQ_LINE.
>
> What kind of conflict do you envision? Pls note level interrupts are
> unsupported ATM.

If KVM_IRQ_LINE asserts a level interrupt and KVM_IRQFD triggers on the
same GSI then the pin is no longer asserted as userspace thinks it is.
Do we just chalk this up to userspace error?

> > Move irqfd to it's own reserved IRQ source ID. Add a capability for
> > userspace to test for this fix.
> >
> > Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> > ---
> >
> > arch/x86/kvm/x86.c | 3 +++
> > include/linux/kvm.h | 1 +
> > include/linux/kvm_host.h | 1 +
> > virt/kvm/eventfd.c | 6 +++---
> > 4 files changed, 8 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 42bce48..cd98673 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -2174,6 +2174,7 @@ 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_IRQ_SOURCE_ID:
> > r = 1;
> > break;
> > case KVM_CAP_COALESCED_MMIO:
> > @@ -6258,6 +6259,8 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
> >
> > /* Reserve bit 0 of irq_sources_bitmap for userspace irq source */
> > set_bit(KVM_USERSPACE_IRQ_SOURCE_ID, &kvm->arch.irq_sources_bitmap);
> > + /* Reserve bit 1 of irq_sources_bitmap for irqfd irq source */
> > + set_bit(KVM_IRQFD_IRQ_SOURCE_ID, &kvm->arch.irq_sources_bitmap);
> >
> > raw_spin_lock_init(&kvm->arch.tsc_write_lock);
> >
> > diff --git a/include/linux/kvm.h b/include/linux/kvm.h
> > index 2ce09aa..ae66b9c 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_IRQ_SOURCE_ID 81
> >
> > #ifdef KVM_CAP_IRQ_ROUTING
> >
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index b70b48b..b763230 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -71,6 +71,7 @@
> > #define KVM_REQ_PMI 17
> >
> > #define KVM_USERSPACE_IRQ_SOURCE_ID 0
> > +#define KVM_IRQFD_IRQ_SOURCE_ID 1
> >
> > struct kvm;
> > struct kvm_vcpu;
>
> Above looks fine but I'm not sure why is the below needed.
> This changes irqfd behaviour for edge GSIs slightly
> in a userspace-visible way. Maybe make it a separate patch
> so it can be considered on merits?

Hmm, the above does nothing without the below. I thought I was just
implementing your idea that IRQFDs should all share a single IRQ source
ID... why is that no longer a good idea? Thanks,

Alex

> > diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
> > index 7d7e2aa..2245cfa 100644
> > --- a/virt/kvm/eventfd.c
> > +++ b/virt/kvm/eventfd.c
> > @@ -67,8 +67,8 @@ irqfd_inject(struct work_struct *work)
> > struct _irqfd *irqfd = container_of(work, struct _irqfd, inject);
> > struct kvm *kvm = irqfd->kvm;
> >
> > - kvm_set_irq(kvm, KVM_USERSPACE_IRQ_SOURCE_ID, irqfd->gsi, 1);
> > - kvm_set_irq(kvm, KVM_USERSPACE_IRQ_SOURCE_ID, irqfd->gsi, 0);
> > + kvm_set_irq(kvm, KVM_IRQFD_IRQ_SOURCE_ID, irqfd->gsi, 1);
> > + kvm_set_irq(kvm, KVM_IRQFD_IRQ_SOURCE_ID, irqfd->gsi, 0);
> > }
> >
> > /*
> > @@ -138,7 +138,7 @@ irqfd_wakeup(wait_queue_t *wait, unsigned mode, int sync, void *key)
> > irq = rcu_dereference(irqfd->irq_entry);
> > /* An event has been signaled, inject an interrupt */
> > if (irq)
> > - kvm_set_msi(irq, kvm, KVM_USERSPACE_IRQ_SOURCE_ID, 1);
> > + kvm_set_msi(irq, kvm, KVM_IRQFD_IRQ_SOURCE_ID, 1);
> > else
> > schedule_work(&irqfd->inject);
> > rcu_read_unlock();