Hi Dmytro, > -----Original Message----- > From: Dmytro Maluka <dmy@xxxxxxxxxxxx> > Sent: Saturday, July 30, 2022 7:35 AM > To: Liu, Rong L <rong.l.liu@xxxxxxxxx>; Christopherson,, Sean > <seanjc@xxxxxxxxxx>; Paolo Bonzini <pbonzini@xxxxxxxxxx>; > kvm@xxxxxxxxxxxxxxx > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>; Ingo Molnar > <mingo@xxxxxxxxxx>; Borislav Petkov <bp@xxxxxxxxx>; Dave Hansen > <dave.hansen@xxxxxxxxxxxxxxx>; x86@xxxxxxxxxx; H. Peter Anvin > <hpa@xxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; Eric Auger > <eric.auger@xxxxxxxxxx>; Alex Williamson > <alex.williamson@xxxxxxxxxx>; Zhenyu Wang > <zhenyuw@xxxxxxxxxxxxxxx>; Tomasz Nowicki <tn@xxxxxxxxxxxx>; > Grzegorz Jaszczyk <jaz@xxxxxxxxxxxx>; Dmitry Torokhov > <dtor@xxxxxxxxxx> > Subject: Re: [PATCH 3/3] KVM: irqfd: Postpone resamplefd notify for > oneshot interrupts > > On 7/29/22 10:48 PM, Liu, Rong L wrote: > > Hi Dmytro, > > > >> -----Original Message----- > >> From: Dmytro Maluka <dmy@xxxxxxxxxxxx> > >> Sent: Tuesday, July 26, 2022 7:08 AM > >> To: Liu, Rong L <rong.l.liu@xxxxxxxxx>; Christopherson,, Sean > >> <seanjc@xxxxxxxxxx>; Paolo Bonzini <pbonzini@xxxxxxxxxx>; > >> kvm@xxxxxxxxxxxxxxx > >> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>; Ingo Molnar > >> <mingo@xxxxxxxxxx>; Borislav Petkov <bp@xxxxxxxxx>; Dave Hansen > >> <dave.hansen@xxxxxxxxxxxxxxx>; x86@xxxxxxxxxx; H. Peter Anvin > >> <hpa@xxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; Eric Auger > >> <eric.auger@xxxxxxxxxx>; Alex Williamson > >> <alex.williamson@xxxxxxxxxx>; Zhenyu Wang > >> <zhenyuw@xxxxxxxxxxxxxxx>; Tomasz Nowicki <tn@xxxxxxxxxxxx>; > >> Grzegorz Jaszczyk <jaz@xxxxxxxxxxxx>; Dmitry Torokhov > >> <dtor@xxxxxxxxxx> > >> Subject: Re: [PATCH 3/3] KVM: irqfd: Postpone resamplefd notify for > >> oneshot interrupts > >> > >> Hi Rong, > >> > >> On 7/26/22 01:44, Liu, Rong L wrote: > >>> Hi Dmytro, > >>> > >>>> -----Original Message----- > >>>> From: Dmytro Maluka <dmy@xxxxxxxxxxxx> > >>>> Sent: Friday, July 15, 2022 8:59 AM > >>>> To: Christopherson,, Sean <seanjc@xxxxxxxxxx>; Paolo Bonzini > >>>> <pbonzini@xxxxxxxxxx>; kvm@xxxxxxxxxxxxxxx > >>>> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>; Ingo Molnar > >>>> <mingo@xxxxxxxxxx>; Borislav Petkov <bp@xxxxxxxxx>; Dave > Hansen > >>>> <dave.hansen@xxxxxxxxxxxxxxx>; x86@xxxxxxxxxx; H. Peter Anvin > >>>> <hpa@xxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; Eric Auger > >>>> <eric.auger@xxxxxxxxxx>; Alex Williamson > >>>> <alex.williamson@xxxxxxxxxx>; Liu, Rong L <rong.l.liu@xxxxxxxxx>; > >>>> Zhenyu Wang <zhenyuw@xxxxxxxxxxxxxxx>; Tomasz Nowicki > >>>> <tn@xxxxxxxxxxxx>; Grzegorz Jaszczyk <jaz@xxxxxxxxxxxx>; > Dmitry > >>>> Torokhov <dtor@xxxxxxxxxx>; Dmytro Maluka > <dmy@xxxxxxxxxxxx> > >>>> 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 > >> > > > > I mean the organization of the code. In current implementation, > > kvm_ioapic_update_eoi_one() calls kvm_notify_acked_irq(), in your > patch, why not > > call kvm_notify_acked_irq() from ioapic_write_indirect() (roughly at > the same > > place where kvm_fire_mask_notifiers is called), instead of calling it > from > > kvm_ioapic_update_eoi_one, since what your intention here is to > notify > > vfio of the end of interrupt at the event of ioapic unmask, instead of > > EOI? > > Ah ok, got your point. > > That was my initial approach in my PoC patch posted in [1]. But then I > dropped it, for 2 reasons: > > 1. Upon feedback from Sean I realized that kvm_notify_acked_irq() is > also doing some other important things besides notifying vfio. In > particular, in irqfd_resampler_ack() we also de-assert the vIRQ via > kvm_set_irq(). In case of IOAPIC it means clearing its bit in IRR > register. If we delay that until unmasking, it means that we change > the way how KVM emulates the interrupt controller. That would seem > inconsistent. > Thanks for clarification. I totally agree that it is important to keep the way how KVM emulates the interrupt controller. > Also kvm_notify_acked_irq() notifies the emulated PIT timer via > kvm_pit_ack_irq(). I haven't analyzed how exactly that PIT stuff > works, so I'm not sure if delaying that until unmask wouldn't cause > any unwanted effects. > > So the idea is to postpone eventfd_signal() only, to fix interaction > with vfio while keeping the rest of the KVM behavior intact. Because > the KVM job is to emulate the interrupt controller (which it already > does correctly), not the external device which is the job of vfio*. > I made a mistake in my last post. I mean just to delay the notification of vfio, but keep the rest of the code as intact as possible. > 2. kvm_notify_acked_irq() can't be called under ioapic->lock, so in [1] > I was unlocking ioapic->lock in ioapic_write_indirect() with a naive > assumption that it was as safe as doing it in > kvm_ioapic_update_eoi_one(). That was probably racy, and I hadn't > figured out how to rework it in a race-free way. > > [1] https://lore.kernel.org/kvm/31420943-8c5f-125c-a5ee- > d2fde2700083@xxxxxxxxxxxx/ > > [*] By "vfio" I always mean "vfio or any other resamplefd user". > > 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@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); > >>>> irqfd->resampler = resampler; > >>>> } > >>>> > >>>> -- > >>>> 2.37.0.170.g444d1eabd0-goog > >>>
