[PATCH v2 4/4] vfio/pci: Allow MMIO regions to be exported through dma-buf

[Christian König]

Am 06.09.22 um 11:51 schrieb Christoph Hellwig:
>> +{
>> +	struct vfio_pci_dma_buf *priv = dmabuf->priv;
>> +	int rc;
>> +
>> +	rc = pci_p2pdma_distance_many(priv->vdev->pdev, &attachment->dev, 1,
>> +				      true);
> This should just use pci_p2pdma_distance.
>
>> +	/*
>> +	 * Since the memory being mapped is a device memory it could never be in
>> +	 * CPU caches.
>> +	 */
> DMA_ATTR_SKIP_CPU_SYNC doesn't even apply to dma_map_resource, not sure
> where this wisdom comes from.
>
>> +	dma_addr = dma_map_resource(
>> +		attachment->dev,
>> +		pci_resource_start(priv->vdev->pdev, priv->index) +
>> +			priv->offset,
>> +		priv->dmabuf->size, dir, DMA_ATTR_SKIP_CPU_SYNC);
> This is not how P2P addresses are mapped.  You need to use
> dma_map_sgtable and have the proper pgmap for it.

The problem is once more that this is MMIO space, in other words 
register BARs which needs to be exported/imported.

Adding struct pages for it generally sounds like the wrong approach 
here. You can't even access this with the CPU or would trigger 
potentially unwanted hardware actions.

Would you mind if I start to tackle this problem?

Regards,
Christian.

>
> The above is just a badly implemented version of the dma-direct
> PCI_P2PDMA_MAP_BUS_ADDR case, ignoring mappings through the host
> bridge or dma-map-ops interactions.