[kvmtool PATCH v9 06/15] PCI: Only allocate IRQ routing entry when available

[André Przywara]

Hi Marc,

thanks for having a look!

On 29/03/17 09:30, Marc Zyngier wrote:
> On 02/02/17 16:32, Andre Przywara wrote:
>> If we need to inject an MSI into the guest, we rely at the moment on a
>> working GSI MSI routing functionality. However we can get away without
>> IRQ routing, if the host supports MSI injection via the KVM_SIGNAL_MSI
>> ioctl.
>> So we try the GSI routing first, but if that fails due to a missing
>> IRQ routing functionality, we fall back to KVM_SIGNAL_MSI (if that is
>> supported).
>>
>> Signed-off-by: Andre Przywara <andre.przywara at arm.com>
>> ---
>>  virtio/pci.c | 16 ++++++++++++++--
>>  1 file changed, 14 insertions(+), 2 deletions(-)
>>
>> diff --git a/virtio/pci.c b/virtio/pci.c
>> index 7cc0ba4..98bf6b7 100644
>> --- a/virtio/pci.c
>> +++ b/virtio/pci.c
>> @@ -193,8 +193,14 @@ static bool virtio_pci__specific_io_out(struct kvm *kvm, struct virtio_device *v
>>  
>>  			gsi = irq__add_msix_route(kvm,
>>  						  &vpci->msix_table[vec].msg);
>> -			if (gsi >= 0)
>> +			if (gsi >= 0) {
>>  				vpci->config_gsi = gsi;
>> +				break;
>> +			}
>> +			if (gsi == -ENXIO &&
>> +			    vpci->features & VIRTIO_PCI_F_SIGNAL_MSI)
>> +					break;
>> +			die("failed to configure MSIs");
>>  			break;
>>  		case VIRTIO_MSI_QUEUE_VECTOR:
>>  			vec = ioport__read16(data);
>> @@ -205,8 +211,14 @@ static bool virtio_pci__specific_io_out(struct kvm *kvm, struct virtio_device *v
>>  
>>  			gsi = irq__add_msix_route(kvm,
>>  						  &vpci->msix_table[vec].msg);
>> -			if (gsi < 0)
>> +			if (gsi < 0) {
>> +				if (gsi == -ENXIO &&
>> +				    vpci->features & VIRTIO_PCI_F_SIGNAL_MSI)
>> +					break;
>> +
>> +				die("failed to configure MSIs");
>>  				break;
>> +			}
>>  			vpci->gsis[vpci->queue_selector] = gsi;
> 
> Is it really expected to find -ENXIO in this array instead of a GSI?

Not sure I get this "in this array" comment here. Any negative return
value ends up in a break, without writing it anywhere.
Later on we check for VIRTIO_PCI_F_SIGNAL_MSI first before referencing
the array. This is admittedly a bit convoluted.
So shall I add comments here? Or rewrite the if statement to make this
more obvious?

Cheers,
Andre.

> 
>>  			if (vdev->ops->notify_vq_gsi)
>>  				vdev->ops->notify_vq_gsi(kvm, vpci->dev,
>>
> 
> Thanks,
> 
> 	M.
>