Re: [PATCH 16/22] vfio/pci.c: use QOM casts where appropriate

["Cédric Le Goater" <clg@redhat.com>]

On 8/29/25 13:07, Mark Cave-Ayland wrote:
> On 15/07/2025 14:38, BALATON Zoltan wrote:
> 
>> On Tue, 15 Jul 2025, Mark Cave-Ayland wrote:
>>> Use QOM casts to convert between VFIOPCIDevice and PCIDevice instead of
>>> accessing pdev directly.
>>>
>>> Signed-off-by: Mark Cave-Ayland <mark.caveayland@nutanix.com>
>>> ---
>>> hw/vfio/pci.c | 202 ++++++++++++++++++++++++++++++--------------------
>>> 1 file changed, 120 insertions(+), 82 deletions(-)
>>>
>>> diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
>>> index 1093b28df7..fb9eb58da5 100644
>>> --- a/hw/vfio/pci.c
>>> +++ b/hw/vfio/pci.c
>>> @@ -119,6 +119,7 @@ static void vfio_intx_mmap_enable(void *opaque)
>>> static void vfio_intx_interrupt(void *opaque)
>>> {
>>>     VFIOPCIDevice *vdev = opaque;
>>> +    PCIDevice *pdev = PCI_DEVICE(vdev);
>>
>> Don't do that. Opaque data is already type checked when it is registered for the callback and cannot be changed so additional type checking here is just a performance hit without any advantage. It's OK to do it in less frequently called functions but don't add unnecessary casts to functions that can be called a lot.
> 
> In general the QOM casts fall into the noise in a standard profile, but I can see how it could be possible they might show up in the interrupt fast path.
> 
> I'll look at getting a vfio-pci perf test set up here to see if there is a noticeable effect in this case, and if so think about what the best approach is.

Booting with 'pci=nomsi' and running iperf should give some info
on INTx performance.

Thanks,

C.