[PATCH v3] vfio/pci: Fix INTx handling on legacy non-PCI 2.3 devices

[Cédric Le Goater]

On 9/22/25 20:21, Timothy Pearson wrote:
> 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. This failure manifests as receiving and
> acknowledging a single interrupt in the guest, while the device continues to
> assert the level interrupt indicating a need for further servicing.
> 
> When lazy IRQ masking is used on DisINTx- (non-PCI 2.3) hardware, the following
> sequence occurs:
> 
>   * Level IRQ assertion on device
>   * IRQ marked disabled in kernel
>   * Host interrupt handler exits without clearing the interrupt on the device
>   * Eventfd is delivered to userspace
>   * Guest processes IRQ and clears device interrupt
>   * Device de-asserts INTx, then re-asserts INTx while the interrupt is masked
>   * Newly asserted interrupt acknowledged by kernel VMM without being handled
>   * Software mask removed by VFIO driver
>   * Device INTx still asserted, host controller does not see new edge after EOI
> 
> The behavior is now platform-dependent.  Some platforms (amd64) will continue
> to spew IRQs for as long as the INTX line remains asserted, therefore the IRQ
> will be handled by the host as soon as the mask is dropped.  Others (ppc64) will
> only send the one request, and if it is not handled no further interrupts will
> be sent.  The former behavior theoretically leaves the system vulnerable to
> interrupt storm, and the latter will result in the device stalling after
> receiving exactly one interrupt in the guest.
> 
> Work around this by disabling lazy IRQ masking for DisINTx- INTx devices.

Timothy,

This changes lacks your SoB.

Thanks,

C.




> ---
>   drivers/vfio/pci/vfio_pci_intrs.c | 7 +++++++
>   1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/vfio/pci/vfio_pci_intrs.c b/drivers/vfio/pci/vfio_pci_intrs.c
> index 123298a4dc8f..61d29f6b3730 100644
> --- a/drivers/vfio/pci/vfio_pci_intrs.c
> +++ b/drivers/vfio/pci/vfio_pci_intrs.c
> @@ -304,9 +304,14 @@ static int vfio_intx_enable(struct vfio_pci_core_device *vdev,
>   
>   	vdev->irq_type = VFIO_PCI_INTX_IRQ_INDEX;
>   
> +	if (!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) {
> +		if (!vdev->pci_2_3)
> +			irq_clear_status_flags(pdev->irq, IRQ_DISABLE_UNLAZY);
>   		vdev->irq_type = VFIO_PCI_NUM_IRQS;
>   		kfree(name);
>   		vfio_irq_ctx_free(vdev, ctx, 0);
> @@ -352,6 +357,8 @@ static void vfio_intx_disable(struct vfio_pci_core_device *vdev)
>   		vfio_virqfd_disable(&ctx->unmask);
>   		vfio_virqfd_disable(&ctx->mask);
>   		free_irq(pdev->irq, ctx);
> +		if (!vdev->pci_2_3)
> +			irq_clear_status_flags(pdev->irq, IRQ_DISABLE_UNLAZY);
>   		if (ctx->trigger)
>   			eventfd_ctx_put(ctx->trigger);
>   		kfree(ctx->name);