Apologies, resent.
----- Original Message -----
> From: "Cédric Le Goater" <[email protected]>
> To: "Timothy Pearson" <[email protected]>, "kvm"
> <[email protected]>
> Cc: "linuxppc-dev" <[email protected]>, "Alex Williamson"
> <[email protected]>
> 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);
