Re: [PATCH 3/3] virtio_pci: Use the DMA API for virtqueues

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Tue, Aug 26, 2014 at 02:17:02PM -0700, Andy Lutomirski wrote:
> A virtqueue is a coherent DMA mapping.  Use the DMA API for it.
> This fixes virtio_pci on Xen.
> 
> Signed-off-by: Andy Lutomirski <luto@amacapital.net>
> ---
>  drivers/virtio/virtio_pci.c | 25 ++++++++++++++++++-------
>  1 file changed, 18 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_pci.c b/drivers/virtio/virtio_pci.c
> index 3d1463c6b120..19039c5bec24 100644
> --- a/drivers/virtio/virtio_pci.c
> +++ b/drivers/virtio/virtio_pci.c
> @@ -80,8 +80,9 @@ struct virtio_pci_vq_info
>  	/* the number of entries in the queue */
>  	int num;
>  
> -	/* the virtual address of the ring queue */
> -	void *queue;
> +	/* the ring queue */
> +	void *queue;			/* virtual address */
> +	dma_addr_t queue_dma_addr;	/* bus address */
>  
>  	/* the list node for the virtqueues list */
>  	struct list_head node;
> @@ -417,15 +418,16 @@ static struct virtqueue *setup_vq(struct virtio_device *vdev, unsigned index,
>  	info->num = num;
>  	info->msix_vector = msix_vec;
>  
> -	size = PAGE_ALIGN(vring_size(num, VIRTIO_PCI_VRING_ALIGN));
> -	info->queue = alloc_pages_exact(size, GFP_KERNEL|__GFP_ZERO);
> +	size = vring_size(num, VIRTIO_PCI_VRING_ALIGN);
> +	info->queue = dma_zalloc_coherent(vdev->dev.parent, size,
> +					  &info->queue_dma_addr, GFP_KERNEL);
>  	if (info->queue == NULL) {
>  		err = -ENOMEM;
>  		goto out_info;
>  	}
>  
>  	/* activate the queue */
> -	iowrite32(virt_to_phys(info->queue) >> VIRTIO_PCI_QUEUE_ADDR_SHIFT,
> +	iowrite32(info->queue_dma_addr >> VIRTIO_PCI_QUEUE_ADDR_SHIFT,
>  		  vp_dev->ioaddr + VIRTIO_PCI_QUEUE_PFN);
>  
>  	/* create the vring */
> @@ -462,7 +464,8 @@ out_assign:
>  	vring_del_virtqueue(vq);
>  out_activate_queue:
>  	iowrite32(0, vp_dev->ioaddr + VIRTIO_PCI_QUEUE_PFN);
> -	free_pages_exact(info->queue, size);
> +	dma_free_coherent(vdev->dev.parent, size,
> +			  info->queue, info->queue_dma_addr);
>  out_info:
>  	kfree(info);
>  	return ERR_PTR(err);
> @@ -493,7 +496,8 @@ static void vp_del_vq(struct virtqueue *vq)
>  	iowrite32(0, vp_dev->ioaddr + VIRTIO_PCI_QUEUE_PFN);
>  
>  	size = PAGE_ALIGN(vring_size(info->num, VIRTIO_PCI_VRING_ALIGN));
> -	free_pages_exact(info->queue, size);
> +	dma_free_coherent(vq->vdev->dev.parent, size,
> +			  info->queue, info->queue_dma_addr);
>  	kfree(info);
>  }
>  
> @@ -712,6 +716,13 @@ static int virtio_pci_probe(struct pci_dev *pci_dev,
>  	if (err)
>  		goto out;
>  
> +	/*
> +	 * We support 64-bit DMA.  If this fails (e.g. some bridge
> +	 * or PV code doesn't or DAC is disabled), then we're okay
> +	 * with 32-bit DMA.

<scratches his head>

I am having a hard time parsing that. Could you expand a bit the
faulting use-case please?

> +	 */
> +	dma_set_mask_and_coherent(&pci_dev->dev, DMA_BIT_MASK(64));

The usual process is:

	ret = dma_set_mask_and_coherent(..)
	if (ret)
		ret = dma_set_mask_and_coherent(.., DMA_BIT_MASK(32))

	if (ret)
		pr_warn("We are truly screwed. Good luck!\n");

> +
>  	err = pci_request_regions(pci_dev, "virtio-pci");
>  	if (err)
>  		goto out_enable_device;
> -- 
> 1.9.3
>