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    SubjectRe: [PATCH v9 1/2] kvm: Use a reserved IRQ source ID for irqfd
    On Tue, Aug 21, 2012 at 02:06:19PM -0600, Alex Williamson wrote:
    > 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?

    Yes: using a level GSI with current irqfd is a userspace error
    because you can lose interrupts anyway.

    Are edge GSIs affected?

    > > > 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.

    Yes. But you can use the above with the new irqfds you are adding.

    > I thought I was just
    > implementing your idea that IRQFDs should all share a single IRQ source
    > ID...

    Sorry I only meant for level irqfds. You are changing edge here.

    > why is that no longer a good idea? Thanks,
    >
    > Alex

    Maybe it is a good idea. I am just asking for the motivation.

    > > > 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();
    >
    >


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