Re: [PATCH] iommu/virtio: Reject IOMMU page granule larger than PAGE_SIZE

[Robin Murphy <robin.murphy-5wv7dgnIgG8@public.gmane.org>]

On 2020-03-18 11:40 am, Jean-Philippe Brucker wrote:
> We don't currently support IOMMUs with a page granule larger than the
> system page size. The IOVA allocator has a BUG_ON() in this case, and
> VFIO has a WARN_ON().
> 
> It might be possible to remove these obstacles if necessary. If the host
> uses 64kB pages and the guest uses 4kB, then a device driver calling
> alloc_page() followed by dma_map_page() will create a 64kB mapping for a
> 4kB physical page, allowing the endpoint to access the neighbouring 60kB
> of memory. This problem could be worked around with bounce buffers.

FWIW the fundamental issue is that callers of iommu_map() may expect to 
be able to map two or more page-aligned regions directly adjacent to 
each other for scatter-gather purposes (or ring buffer tricks), and 
that's just not possible if the IOMMU granule is too big. Bounce 
buffering would be a viable workaround for the streaming DMA API and 
certain similar use-cases, but not in general (e.g. coherent DMA, VFIO, 
GPUs, etc.)

Robin.

> For the moment, rather than triggering the IOVA BUG_ON() on mismatched
> page sizes, abort the virtio-iommu probe with an error message.
> 
> Reported-by: Bharat Bhushan <bbhushan2-eYqpPyKDWXRBDgjK7y7TUQ@public.gmane.org>
> Signed-off-by: Jean-Philippe Brucker <jean-philippe-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
> ---
>   drivers/iommu/virtio-iommu.c | 9 +++++++++
>   1 file changed, 9 insertions(+)
> 
> diff --git a/drivers/iommu/virtio-iommu.c b/drivers/iommu/virtio-iommu.c
> index 6d4e3c2a2ddb..80d5d8f621ab 100644
> --- a/drivers/iommu/virtio-iommu.c
> +++ b/drivers/iommu/virtio-iommu.c
> @@ -998,6 +998,7 @@ static int viommu_probe(struct virtio_device *vdev)
>   	struct device *parent_dev = vdev->dev.parent;
>   	struct viommu_dev *viommu = NULL;
>   	struct device *dev = &vdev->dev;
> +	unsigned long viommu_page_size;
>   	u64 input_start = 0;
>   	u64 input_end = -1UL;
>   	int ret;
> @@ -1028,6 +1029,14 @@ static int viommu_probe(struct virtio_device *vdev)
>   		goto err_free_vqs;
>   	}
>   
> +	viommu_page_size = 1UL << __ffs(viommu->pgsize_bitmap);
> +	if (viommu_page_size > PAGE_SIZE) {
> +		dev_err(dev, "granule 0x%lx larger than system page size 0x%lx\n",
> +			viommu_page_size, PAGE_SIZE);
> +		ret = -EINVAL;
> +		goto err_free_vqs;
> +	}
> +
>   	viommu->map_flags = VIRTIO_IOMMU_MAP_F_READ | VIRTIO_IOMMU_MAP_F_WRITE;
>   	viommu->last_domain = ~0U;
>   
>