vp_vdpa: don't allocate unused msix vectors

When there is a ctlq and it doesn't require interrupt
callbacks,the original method of calculating vectors
wastes hardware msi or msix resources as well as system
IRQ resources.

When conducting performance testing using testpmd in the
guest os, it was found that the performance was lower compared
to directly using vfio-pci to passthrough the device

In scenarios where the virtio device in the guest os does
not utilize interrupts, the vdpa driver still configures
the hardware's msix vector. Therefore, the hardware still
sends interrupts to the host os. Because of this unnecessary
action by the hardware, hardware performance decreases, and
it also affects the performance of the host os.

Before modification:(interrupt mode)
32: 0 0 0 0 PCI-MSI 32768-edge vp-vdpa[0000:00:02.0]-0
33: 0 0 0 0 PCI-MSI 32769-edge vp-vdpa[0000:00:02.0]-1
34: 0 0 0 0 PCI-MSI 32770-edge vp-vdpa[0000:00:02.0]-2
35: 0 0 0 0 PCI-MSI 32771-edge vp-vdpa[0000:00:02.0]-config

After modification:(interrupt mode)
32: 0 0 1 7 PCI-MSI 32768-edge vp-vdpa[0000:00:02.0]-0
33: 36 0 3 0 PCI-MSI 32769-edge vp-vdpa[0000:00:02.0]-1
34: 0 0 0 0 PCI-MSI 32770-edge vp-vdpa[0000:00:02.0]-config

Before modification:(virtio pmd mode for guest os)
32: 0 0 0 0 PCI-MSI 32768-edge vp-vdpa[0000:00:02.0]-0
33: 0 0 0 0 PCI-MSI 32769-edge vp-vdpa[0000:00:02.0]-1
34: 0 0 0 0 PCI-MSI 32770-edge vp-vdpa[0000:00:02.0]-2
35: 0 0 0 0 PCI-MSI 32771-edge vp-vdpa[0000:00:02.0]-config

After modification:(virtio pmd mode for guest os)
32: 0 0 0 0 PCI-MSI 32768-edge vp-vdpa[0000:00:02.0]-config

To verify the use of the virtio PMD mode in the guest operating
system, the following patch needs to be applied to QEMU:
https://lore.kernel.org/all/[e-mail]

Signed-off-by: Yuxue Liu <[e-mail]>
Acked-by: Jason Wang <[e-mail]>
Reviewed-by: Heng Qi <[e-mail]>
Message-Id: <[e-mail]>
Signed-off-by: Michael S. Tsirkin <[e-mail]>

drivers/vdpa/virtio_pci/vp_vdpa.c

···
 	struct pci_dev *pdev = mdev->pci_dev;
 	int i, ret, irq;
 	int queues = vp_vdpa->queues;
-	int vectors = queues + 1;
+	int vectors = 1;
+	int msix_vec = 0;
+
+	for (i = 0; i < queues; i++) {
+		if (vp_vdpa->vring[i].cb.callback)
+			vectors++;
+	}
 
 	ret = pci_alloc_irq_vectors(pdev, vectors, vectors, PCI_IRQ_MSIX);
 	if (ret != vectors) {
···
 	vp_vdpa->vectors = vectors;
 
 	for (i = 0; i < queues; i++) {
+		if (!vp_vdpa->vring[i].cb.callback)
+			continue;
+
 		snprintf(vp_vdpa->vring[i].msix_name, VP_VDPA_NAME_SIZE,
 			"vp-vdpa[%s]-%d\n", pci_name(pdev), i);
-		irq = pci_irq_vector(pdev, i);
+		irq = pci_irq_vector(pdev, msix_vec);
 		ret = devm_request_irq(&pdev->dev, irq,
 				       vp_vdpa_vq_handler,
 				       0, vp_vdpa->vring[i].msix_name,
···
 				"vp_vdpa: fail to request irq for vq %d\n", i);
 			goto err;
 		}
-		vp_modern_queue_vector(mdev, i, i);
+		vp_modern_queue_vector(mdev, i, msix_vec);
 		vp_vdpa->vring[i].irq = irq;
+		msix_vec++;
 	}
 
 	snprintf(vp_vdpa->msix_name, VP_VDPA_NAME_SIZE, "vp-vdpa[%s]-config\n",
 		 pci_name(pdev));
-	irq = pci_irq_vector(pdev, queues);
+	irq = pci_irq_vector(pdev, msix_vec);
 	ret = devm_request_irq(&pdev->dev, irq,	vp_vdpa_config_handler, 0,
 			       vp_vdpa->msix_name, vp_vdpa);
 	if (ret) {
 		dev_err(&pdev->dev,
-			"vp_vdpa: fail to request irq for vq %d\n", i);
+			"vp_vdpa: fail to request irq for config: %d\n", ret);
 			goto err;
 	}
-	vp_modern_config_vector(mdev, queues);
+	vp_modern_config_vector(mdev, msix_vec);
 	vp_vdpa->config_irq = irq;
 
 	return 0;