On 7/15/22 17:59, Dmytro Maluka wrote: > The existing KVM mechanism for forwarding of level-triggered interrupts > using resample eventfd doesn't work quite correctly in the case of > interrupts that are handled in a Linux guest as oneshot interrupts > (IRQF_ONESHOT). Such an interrupt is acked to the device in its > threaded irq handler, i.e. later than it is acked to the interrupt > controller (EOI at the end of hardirq), not earlier. > > Linux keeps such interrupt masked until its threaded handler finishes, > to prevent the EOI from re-asserting an unacknowledged interrupt. > However, with KVM + vfio (or whatever is listening on the resamplefd) > we don't check that the interrupt is still masked in the guest at the > moment of EOI. Resamplefd is notified regardless, so vfio prematurely > unmasks the host physical IRQ, thus a new (unwanted) physical interrupt > is generated in the host and queued for injection to the guest. > > The fact that the virtual IRQ is still masked doesn't prevent this new > physical IRQ from being propagated to the guest, because: > > 1. It is not guaranteed that the vIRQ will remain masked by the time > when vfio signals the trigger eventfd. > 2. KVM marks this IRQ as pending (e.g. setting its bit in the virtual > IRR register of IOAPIC on x86), so after the vIRQ is unmasked, this > new pending interrupt is injected by KVM to the guest anyway. > > There are observed at least 2 user-visible issues caused by those > extra erroneous pending interrupts for oneshot irq in the guest: > > 1. System suspend aborted due to a pending wakeup interrupt from > ChromeOS EC (drivers/platform/chrome/cros_ec.c). > 2. Annoying "invalid report id data" errors from ELAN0000 touchpad > (drivers/input/mouse/elan_i2c_core.c), flooding the guest dmesg > every time the touchpad is touched. > > This patch fixes the issue on x86 by checking if the interrupt is > unmasked when we receive irq ack (EOI) and, in case if it's masked, > postponing resamplefd notify until the guest unmasks it. > > Important notes: > > 1. It doesn't fix the issue for other archs yet, due to some missing > KVM functionality needed by this patch: > - calling mask notifiers is implemented for x86 only > - irqchip ->is_masked() is implemented for x86 only > > 2. It introduces an additional spinlock locking in the resample notify > path, since we are no longer just traversing an RCU list of irqfds > but also updating the resampler state. Hopefully this locking won't > noticeably slow down anything for anyone. > > Regarding #2, there may be an alternative solution worth considering: > extend KVM irqfd (userspace) API to send mask and unmask notifications > directly to vfio/whatever, in addition to resample notifications, to > let vfio check the irq state on its own. There is already locking on > vfio side (see e.g. vfio_platform_unmask()), so this way we would avoid > introducing any additional locking. Also such mask/unmask notifications > could be useful for other cases. > > Link: https://lore.kernel.org/kvm/31420943-8c5f-125c-a5ee-d2fde2700083@xxxxxxxxxxxx/ > Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx> > Signed-off-by: Dmytro Maluka <dmy@xxxxxxxxxxxx> > --- > include/linux/kvm_irqfd.h | 14 ++++++++++++ > virt/kvm/eventfd.c | 45 +++++++++++++++++++++++++++++++++++++++ > 2 files changed, 59 insertions(+) > > diff --git a/include/linux/kvm_irqfd.h b/include/linux/kvm_irqfd.h > index dac047abdba7..01754a1abb9e 100644 > --- a/include/linux/kvm_irqfd.h > +++ b/include/linux/kvm_irqfd.h > @@ -19,6 +19,16 @@ > * resamplefd. All resamplers on the same gsi are de-asserted > * together, so we don't need to track the state of each individual > * user. We can also therefore share the same irq source ID. > + * > + * A special case is when the interrupt is still masked at the moment > + * an irq ack is received. That likely means that the interrupt has > + * been acknowledged to the interrupt controller but not acknowledged > + * to the device yet, e.g. it might be a Linux guest's threaded > + * oneshot interrupt (IRQF_ONESHOT). In this case notifying through > + * resamplefd is postponed until the guest unmasks the interrupt, > + * which is detected through the irq mask notifier. This prevents > + * erroneous extra interrupts caused by premature re-assert of an > + * unacknowledged interrupt by the resamplefd listener. > */ > struct kvm_kernel_irqfd_resampler { > struct kvm *kvm; > @@ -28,6 +38,10 @@ struct kvm_kernel_irqfd_resampler { > */ > struct list_head list; > struct kvm_irq_ack_notifier notifier; > + struct kvm_irq_mask_notifier mask_notifier; > + bool masked; > + bool pending; > + spinlock_t lock; > /* > * Entry in list of kvm->irqfd.resampler_list. Use for sharing > * resamplers among irqfds on the same gsi. > diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c > index 50ddb1d1a7f0..9ff47ac33790 100644 > --- a/virt/kvm/eventfd.c > +++ b/virt/kvm/eventfd.c > @@ -75,6 +75,44 @@ irqfd_resampler_ack(struct kvm_irq_ack_notifier *kian) > kvm_set_irq(kvm, KVM_IRQFD_RESAMPLE_IRQ_SOURCE_ID, > resampler->notifier.gsi, 0, false); > > + spin_lock(&resampler->lock); > + if (resampler->masked) { > + resampler->pending = true; > + spin_unlock(&resampler->lock); > + return; > + } > + spin_unlock(&resampler->lock); > + > + idx = srcu_read_lock(&kvm->irq_srcu); > + > + list_for_each_entry_srcu(irqfd, &resampler->list, resampler_link, > + srcu_read_lock_held(&kvm->irq_srcu)) > + eventfd_signal(irqfd->resamplefd, 1); > + > + srcu_read_unlock(&kvm->irq_srcu, idx); > +} > + > +static void > +irqfd_resampler_mask(struct kvm_irq_mask_notifier *kimn, bool masked) > +{ > + struct kvm_kernel_irqfd_resampler *resampler; > + struct kvm *kvm; > + struct kvm_kernel_irqfd *irqfd; > + int idx; > + > + resampler = container_of(kimn, > + struct kvm_kernel_irqfd_resampler, mask_notifier); > + kvm = resampler->kvm; > + > + spin_lock(&resampler->lock); > + resampler->masked = masked; > + if (masked || !resampler->pending) { > + spin_unlock(&resampler->lock); > + return; > + } > + resampler->pending = false; > + spin_unlock(&resampler->lock); > + > idx = srcu_read_lock(&kvm->irq_srcu); > > list_for_each_entry_srcu(irqfd, &resampler->list, resampler_link, > @@ -98,6 +136,8 @@ irqfd_resampler_shutdown(struct kvm_kernel_irqfd *irqfd) > if (list_empty(&resampler->list)) { > list_del(&resampler->link); > kvm_unregister_irq_ack_notifier(kvm, &resampler->notifier); > + kvm_unregister_irq_mask_notifier(kvm, resampler->mask_notifier.irq, > + &resampler->mask_notifier); > kvm_set_irq(kvm, KVM_IRQFD_RESAMPLE_IRQ_SOURCE_ID, > resampler->notifier.gsi, 0, false); > kfree(resampler); > @@ -367,11 +407,16 @@ kvm_irqfd_assign(struct kvm *kvm, struct kvm_irqfd *args) > INIT_LIST_HEAD(&resampler->list); > resampler->notifier.gsi = irqfd->gsi; > resampler->notifier.irq_acked = irqfd_resampler_ack; > + resampler->mask_notifier.func = irqfd_resampler_mask; > + kvm_irq_is_masked(kvm, irqfd->gsi, &resampler->masked); > + spin_lock_init(&resampler->lock); > INIT_LIST_HEAD(&resampler->link); > > list_add(&resampler->link, &kvm->irqfds.resampler_list); > kvm_register_irq_ack_notifier(kvm, > &resampler->notifier); > + kvm_register_irq_mask_notifier(kvm, irqfd->gsi, > + &resampler->mask_notifier); I realized this is a bit racy: we may miss a mask or unmask event just before kvm_register_irq_ack_notifier(), so irqfd_resampler_ack() may see an outdated irq mask state. Moving kvm_register_irq_mask_notifier() before kvm_register_irq_ack_notifier() isn't enough, since a mask or unmask may still happen just before kvm_register_irq_mask_notifier(). This race can be avoided by moving also resampler->masked initialization (kvm_irq_is_masked()) after kvm_register_irq_mask_notifier(). But then kvm_irq_is_masked() would need to be called under resampler->lock, which could cause a deadlock, since kvm_irq_is_masked() locks ioapic->lock while irqfd_resampler_mask() is called under ioapic->lock (or correspondingly pic->lock). So for v2 I'm considering replacing kvm_irq_is_masked() with a function like the following, for registering & initializing mask notifier in a presumably race-free deadlock-free way. (The below is just a sketch showing the locking order; the actual code would be more complicated due to irq -> pin mapping etc.) void kvm_register_and_fire_irq_mask_notifier(struct kvm *kvm, int irq, struct kvm_irq_mask_notifier *kimn) { struct kvm_pic *pic = kvm->arch.vpic; struct kvm_ioapic *ioapic = kvm->arch.vioapic; bool masked; mutex_lock(&kvm->irq_lock); spin_lock(&pic->lock); spin_lock(&ioapic->lock); kimn->irq = irq; hlist_add_head_rcu(&kimn->link, &kvm->arch.mask_notifier_list); masked = kvm_pic_irq_is_masked(pic, irq) && kvm_ioapic_irq_is_masked(ioapic, irq); kimn->func(kimn, masked); spin_unlock(&ioapic->lock); spin_unlock(&pic->lock); mutex_unlock(&kvm->irq_lock); } This implies that I'll probably go with moving the mask notifiers stuff back to x86-specific code, and just add a weak no-op version for other architectures. Thanks, Dmytro > irqfd->resampler = resampler; > } >
