KVM: x86/ioapic: Resample the pending state of an IRQ when unmasking

KVM irqfd based emulation of level-triggered interrupts doesn't work
quite correctly in some cases, particularly 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 always notify resamplefd at the EOI, so vfio prematurely unmasks the
host physical IRQ, thus a new physical interrupt is fired in the host.
This extra interrupt in the host is not a problem per se. The problem is
that it is unconditionally queued for injection into the guest, so the
guest sees an extra bogus interrupt. [*]

There are observed at least 2 user-visible issues caused by those
extra erroneous interrupts for a 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.

The core issue here is that by the time when the guest unmasks the IRQ,
the physical IRQ line is no longer asserted (since the guest has
acked the interrupt to the device in the meantime), yet we
unconditionally inject the interrupt queued into the guest by the
previous resampling. So to fix the issue, we need a way to detect that
the IRQ is no longer pending, and cancel the queued interrupt in this
case.

With IOAPIC we are not able to probe the physical IRQ line state
directly (at least not if the underlying physical interrupt controller
is an IOAPIC too), so in this patch we use irqfd resampler for that.
Namely, instead of injecting the queued interrupt, we just notify the
resampler that this interrupt is done. If the IRQ line is actually
already deasserted, we are done. If it is still asserted, a new
interrupt will be shortly triggered through irqfd and injected into the
guest.

In the case if there is no irqfd resampler registered for this IRQ, we
cannot fix the issue, so we keep the existing behavior: immediately
unconditionally inject the queued interrupt.

This patch fixes the issue for x86 IOAPIC only. In the long run, we can
fix it for other irqchips and other architectures too, possibly taking
advantage of reading the physical state of the IRQ line, which is
possible with some other irqchips (e.g. with arm64 GIC, maybe even with
the legacy x86 PIC).

[*] In this description we assume that the interrupt is a physical host
    interrupt forwarded to the guest e.g. by vfio. Potentially the same
    issue may occur also with a purely virtual interrupt from an
    emulated device, e.g. if the guest handles this interrupt, again, as
    a oneshot interrupt.

Signed-off-by: Dmytro Maluka <dmy@semihalf.com>
Link: https://lore.kernel.org/kvm/31420943-8c5f-125c-a5ee-d2fde2700083@semihalf.com/
Link: https://lore.kernel.org/lkml/87o7wrug0w.wl-maz@kernel.org/
Message-Id: <20230322204344.50138-3-dmy@semihalf.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 33 insertions, 3 deletions

diff --git a/arch/x86/kvm/ioapic.c b/arch/x86/kvm/ioapic.c
index 042dee556125..995eb5054360 100644
--- a/arch/x86/kvm/ioapic.c
+++ b/arch/x86/kvm/ioapic.c
@@ -368,9 +368,39 @@ static void ioapic_write_indirect(struct kvm_ioapic *ioapic, u32 val)
 		mask_after = e->fields.mask;
 		if (mask_before != mask_after)
 			kvm_fire_mask_notifiers(ioapic->kvm, KVM_IRQCHIP_IOAPIC, index, mask_after);
-		if (e->fields.trig_mode == IOAPIC_LEVEL_TRIG
-		    && ioapic->irr & (1 << index))
-			ioapic_service(ioapic, index, false);
+		if (e->fields.trig_mode == IOAPIC_LEVEL_TRIG &&
+		    ioapic->irr & (1 << index) && !e->fields.mask && !e->fields.remote_irr) {
+			/*
+			 * Pending status in irr may be outdated: the IRQ line may have
+			 * already been deasserted by a device while the IRQ was masked.
+			 * This occurs, for instance, if the interrupt is handled in a
+			 * Linux guest as a oneshot interrupt (IRQF_ONESHOT). In this
+			 * case the guest acknowledges the interrupt to the device in
+			 * its threaded irq handler, i.e. after the EOI but before
+			 * unmasking, so at the time of unmasking the IRQ line is
+			 * already down but our pending irr bit is still set. In such
+			 * cases, injecting this pending interrupt to the guest is
+			 * buggy: the guest will receive an extra unwanted interrupt.
+			 *
+			 * So we need to check here if the IRQ is actually still pending.
+			 * As we are generally not able to probe the IRQ line status
+			 * directly, we do it through irqfd resampler. Namely, we clear
+			 * the pending status and notify the resampler that this interrupt
+			 * is done, without actually injecting it into the guest. If the
+			 * IRQ line is actually already deasserted, we are done. If it is
+			 * still asserted, a new interrupt will be shortly triggered
+			 * through irqfd and injected into the guest.
+			 *
+			 * If, however, it's not possible to resample (no irqfd resampler
+			 * registered for this irq), then unconditionally inject this
+			 * pending interrupt into the guest, so the guest will not miss
+			 * an interrupt, although may get an extra unwanted interrupt.
+			 */
+			if (kvm_notify_irqfd_resampler(ioapic->kvm, KVM_IRQCHIP_IOAPIC, index))
+				ioapic->irr &= ~(1 << index);
+			else
+				ioapic_service(ioapic, index, false);
+		}
 		if (e->fields.delivery_mode == APIC_DM_FIXED) {
 			struct kvm_lapic_irq irq;