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

From: Auger Eric <eric.auger@redhat.com>
To: Jean-Philippe Brucker <jean-philippe@linaro.org>,
	iommu@lists.linux-foundation.org,
	virtualization@lists.linux-foundation.org
Cc: robin.murphy@arm.com, Bharat Bhushan <bbhushan2@marvell.com>
Subject: Re: [PATCH] iommu/virtio: Reject IOMMU page granule larger than PAGE_SIZE
Date: Wed, 25 Mar 2020 22:05:14 +0100	[thread overview]
Message-ID: <8305aadc-ee0e-382d-b605-b6cfce98b2ac@redhat.com> (raw)
In-Reply-To: <20200318114047.1518048-1-jean-philippe@linaro.org>

Hi Jean,

On 3/18/20 12:40 PM, 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.
> 
> 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@marvell.com>
> Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.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);
Given the fact we now head towards probing the device for its
page_size_mask in viommu_add_device() the check may need to happen
later, in viommu_domain_finalise() for instance?

Thanks

Eric
> +	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;
>  
> 

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