Re: [PATCH v5] vp_vdpa: don't allocate unused msix vectors

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Apr 10, 2024 at 11:30:20AM +0800, lyx634449800 wrote:
> From: Yuxue Liu <yuxue.liu@jaguarmicro.com>
> 
> 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.

I just have a question on this part. How come hardware
sends interrupts does not guest driver disable them?

> 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/20240408073311.2049-1-yuxue.liu@jaguarmicro.com
> 
> Signed-off-by: Yuxue Liu <yuxue.liu@jaguarmicro.com>
> Acked-by: Jason Wang <jasowang@redhat.com>
> Reviewed-by: Heng Qi <hengqi@linux.alibaba.com>
> ---
> V5: modify the description of the printout when an exception occurs
> V4: update the title and assign values to uninitialized variables
> V3: delete unused variables and add validation records
> V2: fix when allocating IRQs, scan all queues
> 
>  drivers/vdpa/virtio_pci/vp_vdpa.c | 22 ++++++++++++++++------
>  1 file changed, 16 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/vdpa/virtio_pci/vp_vdpa.c b/drivers/vdpa/virtio_pci/vp_vdpa.c
> index df5f4a3bccb5..8de0224e9ec2 100644
> --- a/drivers/vdpa/virtio_pci/vp_vdpa.c
> +++ b/drivers/vdpa/virtio_pci/vp_vdpa.c
> @@ -160,7 +160,13 @@ static int vp_vdpa_request_irq(struct vp_vdpa *vp_vdpa)
>  	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) {
> @@ -173,9 +179,12 @@ static int vp_vdpa_request_irq(struct vp_vdpa *vp_vdpa)
>  	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,
> @@ -185,21 +194,22 @@ static int vp_vdpa_request_irq(struct vp_vdpa *vp_vdpa)
>  				"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;
> -- 
> 2.43.0