[PATCH] vfio/pci: Fix INTx handling on legacy DisINTx- PCI devices

[Timothy Pearson]

PCI devices prior to PCI 2.3 both use level interrupts and do not support
interrupt masking, leading to a failure when passed through to a KVM guest on
at least the ppc64 platform, which does not utilize the resample IRQFD. This
failure manifests as receiving and acknowledging a single interrupt in the guest
while leaving the host physical device VFIO IRQ pending.

Level interrupts in general require special handling due to their inherently
asynchronous nature; both the host and guest interrupt controller need to
remain in synchronization in order to coordinate mask and unmask operations.
When lazy IRQ masking is used on DisINTx- hardware, the following sequence
occurs:

 * Level IRQ assertion on host
 * IRQ trigger within host interrupt controller, routed to VFIO driver
 * Host EOI with hardware level IRQ still asserted
 * Software mask of interrupt source by VFIO driver
 * Generation of event and IRQ trigger in KVM guest interrupt controller
 * Level IRQ deassertion on host
 * Guest EOI
 * Guest IRQ level deassertion
 * Removal of software mask by VFIO driver

Note that no actual state change occurs within the host interrupt controller,
unlike what would happen with either DisINTx+ hardware or message interrupts.
The host EOI is not fired with the hardware level IRQ deasserted, and the
level interrupt is not re-armed within the host interrupt controller, leading
to an unrecoverable stall of the device.

Work around this by disabling lazy IRQ masking for DisINTx- INTx devices.

---
 drivers/vfio/pci/vfio_pci_intrs.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/vfio/pci/vfio_pci_intrs.c b/drivers/vfio/pci/vfio_pci_intrs.c
index 123298a4dc8f..011169ca7a34 100644
--- a/drivers/vfio/pci/vfio_pci_intrs.c
+++ b/drivers/vfio/pci/vfio_pci_intrs.c
@@ -304,6 +304,9 @@ static int vfio_intx_enable(struct vfio_pci_core_device *vdev,
 
 	vdev->irq_type = VFIO_PCI_INTX_IRQ_INDEX;
 
+	if (is_intx(vdev) && !vdev->pci_2_3)
+		irq_set_status_flags(pdev->irq, IRQ_DISABLE_UNLAZY);
+
 	ret = request_irq(pdev->irq, vfio_intx_handler,
 			  irqflags, ctx->name, ctx);
 	if (ret) {
@@ -351,6 +354,8 @@ static void vfio_intx_disable(struct vfio_pci_core_device *vdev)
 	if (ctx) {
 		vfio_virqfd_disable(&ctx->unmask);
 		vfio_virqfd_disable(&ctx->mask);
+		if (!vdev->pci_2_3)
+			irq_clear_status_flags(pdev->irq, IRQ_DISABLE_UNLAZY);
 		free_irq(pdev->irq, ctx);
 		if (ctx->trigger)
 			eventfd_ctx_put(ctx->trigger);
-- 
2.39.5