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

[Auger Eric <eric.auger@redhat.com>]

Hi,

On 3/18/20 1:00 PM, Robin Murphy wrote:
> 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().

Adding Alex in CC in case he has time to jump in. At the moment I don't
get why this WARN_ON() is here.

This was introduced in
c8dbca165bb090f926996a572ea2b5b577b34b70 vfio/iommu_type1: Avoid overflow

>>
>> 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.

I understand this is a introduced as a temporary solution but this
sounds as an important limitation to me. For instance this will prevent
from running a fedora guest exposed with a virtio-iommu with a RHEL host.

Thanks

Eric
>>
>> 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);
>> +    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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