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

[Timothy Pearson]

Apologies, resent.

----- Original Message -----
> From: "Cédric Le Goater" <clg at redhat.com>
> To: "Timothy Pearson" <tpearson at raptorengineering.com>, "kvm" <kvm at vger.kernel.org>
> Cc: "linuxppc-dev" <linuxppc-dev at lists.ozlabs.org>, "Alex Williamson" <alex.williamson at redhat.com>
> Sent: Tuesday, September 23, 2025 12:02:12 PM
> Subject: Re: [PATCH v3] vfio/pci: Fix INTx handling on legacy non-PCI 2.3 devices

> 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);