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SubjectRe: [PATCH 3/3] KVM: irqfd: Postpone resamplefd notify for oneshot interrupts
From
Hi Rong,

On 7/26/22 01:44, Liu, Rong L wrote:
> Hi Dmytro,
>
>> -----Original Message-----
>> From: Dmytro Maluka <dmy@semihalf.com>
>> Sent: Friday, July 15, 2022 8:59 AM
>> To: Christopherson,, Sean <seanjc@google.com>; Paolo Bonzini
>> <pbonzini@redhat.com>; kvm@vger.kernel.org
>> Cc: Thomas Gleixner <tglx@linutronix.de>; Ingo Molnar
>> <mingo@redhat.com>; Borislav Petkov <bp@alien8.de>; Dave Hansen
>> <dave.hansen@linux.intel.com>; x86@kernel.org; H. Peter Anvin
>> <hpa@zytor.com>; linux-kernel@vger.kernel.org; Eric Auger
>> <eric.auger@redhat.com>; Alex Williamson
>> <alex.williamson@redhat.com>; Liu, Rong L <rong.l.liu@intel.com>;
>> Zhenyu Wang <zhenyuw@linux.intel.com>; Tomasz Nowicki
>> <tn@semihalf.com>; Grzegorz Jaszczyk <jaz@semihalf.com>; Dmitry
>> Torokhov <dtor@google.com>; Dmytro Maluka <dmy@semihalf.com>
>> Subject: [PATCH 3/3] KVM: irqfd: Postpone resamplefd notify for oneshot
>> interrupts
>>
>> 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.
>>
>
> Instead of using a spinlock waiting for the unmask event, is it possible to call
> resampler notify directly when unmask event happens, instead of calling it on
> EOI?

In this patch, resampler notify is already called directly when unmask
happens: e.g. with IOAPIC, when the guest unmasks the interrupt by
writing to IOREDTBLx register, ioapic_write_indirect() calls
kvm_fire_mask_notifiers() which calls irqfd_resampler_mask() which
notifies the resampler. On EOI we postpone it just by setting
resampler->pending to true, not by waiting. The spinlock is needed
merely to synchronize reading & updating resampler->pending and
resampler->masked values between possibly concurrently running instances
of irqfd_resampler_ack() and/or irqfd_resampler_mask().

Thanks,
Dmytro

>
>> 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@semihalf.com/
>> Suggested-by: Sean Christopherson <seanjc@google.com>
>> Signed-off-by: Dmytro Maluka <dmy@semihalf.com>
>> ---
>> 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);
>> irqfd->resampler = resampler;
>> }
>>
>> --
>> 2.37.0.170.g444d1eabd0-goog
>

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