Re: [Qemu-devel] [RFC PATCH v2] kvm: x86: ignore ioapic polarity

[Paolo Bonzini]

Il 28/02/2014 05:06, Gabriel L. Somlo ha scritto:
> +NOTE: For each level-triggered interrupt managed by a virtual ioapic,
> +the guest OS may set a polarity value (bit 13 of each corresponding I/O
> +redirection table register). The polarity bit defines the relationship
> +between an irq line's logical state (active/asserted = 1, inactive = 0)
> +and its physical state (high = 1, low = 0). When the polarity bit is 0
> +(ActiveHigh), logical and physical states are matched (active == high == 1,
> +inactive == low == 0). When polarity is set to 1 (ActiveLow), logical and
> +physical state are opposites (logical == !physical). Typically, guests are
> +expected to use the same polarity across all level-triggered interrupts,
> +as directed by ACPI. Historically, both KVM and QEMU have accumulated a
> +significant number of optimizations based on the hard-coded assumption
> +that polarity will always be set to ActiveHigh. When interfacing with KVM,
> +QEMU will use the ActiveHigh convention for all level-triggered irqs,
> +regardless of how the guest OS has configured the polarity bits in the
> +ioapic registers. As such, KVM must also ignore these bits, and always act
> +as if the logical and physical states of an irq line exactly match each
> +other (i.e., follow the ActiveHigh convention regardless of polarity).
> +

Instead of this, I'm adding the following to the KVM_IRQ_LINE ioctl:

+On real hardware, interrupt pins can be active-low or active-high.  This
+does not matter for the level field of struct kvm_irq_level: 1 always
+means active (asserted), 0 means inactive (deasserted).
+
+x86 allows the operating system to program the interrupt polarity
+(active-low/active-high) for level-triggered interrupts, and KVM used
+to consider the polarity.  However, due to bitrot in the handling of
+active-low interrupts, the above convention is now valid on x86 too.
+This is signaled by KVM_CAP_X86_IOAPIC_POLARITY_IGNORED.  Userspace
+should not present interrupts to the guest as active-low unless this
+capability is present (or unless it is not using the in-kernel irqchip,
+of course).

and applying the patch to kvm/queue.

Paolo