fix(pci): configure legacy interrupts level triggered and active low

A legacy PCI interrupt is signalled level triggered and active low,
while the IOAPIC defaults to the edge triggered, active high behavior of
the ISA interrupts. A passed-through device therefore stops delivering
interrupts after its first one, because the host only unmasks it once
the guest has completed a level triggered interrupt.

Correct the redirection entry where a legacy interrupt line is handed
out.

Author: stlankes

src/arch/x86_64/kernel/apic.rs

@@ -613,6 +613,25 @@ fn ioapic_set_interrupt(irq: u8, apicid: u8, enabled: bool) {
 	ioapic_write(IOAPIC_REG_TABLE + off + 1, ioredirect_upper);
 }
 
+/// Reconfigures an interrupt line for a legacy PCI interrupt.
+///
+/// [`init_ioapic`] sets up every line for the edge triggered, active high
+/// behavior of the ISA interrupts. PCI signals its interrupts level triggered
+/// and active low instead.
+pub(crate) fn ioapic_set_pci_interrupt(irq: u8, apicid: u8) {
+	assert!(irq <= 24);
+
+	let off = u32::from(irq * 2);
+	let ioredirect_upper = u32::from(apicid) << 24;
+	// Vector, active low (bit 13) and level triggered (bit 15).
+	let ioredirect_lower = u32::from(0x20 + irq) | (1 << 13) | (1 << 15);
+
+	debug!("Configuring irq {irq} as level triggered, active low");
+
+	ioapic_write(IOAPIC_REG_TABLE + off, ioredirect_lower);
+	ioapic_write(IOAPIC_REG_TABLE + off + 1, ioredirect_upper);
+}
+
 pub fn init_local_apic() {
 	// Mask out all interrupts we don't need right now.
 	local_apic_write(IA32_X2APIC_LVT_TIMER, APIC_LVT_MASK);

src/drivers/pci.rs

@@ -181,6 +181,15 @@ impl<T: ConfigRegionAccess> PciDevice<T> {
 					return None;
 				}
 
+				// A legacy PCI interrupt is signaled level triggered and active
+				// low, while the IOAPIC defaults to the edge triggered, active
+				// high behavior of the ISA interrupts. Without the correction a
+				// passed-through device stops delivering interrupts after its
+				// first one, because the host only unmasks the interrupt once the
+				// guest has completed a level triggered one.
+				#[cfg(target_arch = "x86_64")]
+				crate::arch::kernel::apic::ioapic_set_pci_interrupt(line, 0);
+
 				Some(line)
 			}
 			5.. => {