On Wed, 18 Mar 2020 17:14:05 +0100
Auger Eric <eric.auger@xxxxxxxxxx> wrote:
> 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
I don't recall if I had something specific in mind when adding this
warning, but if PAGE_SIZE is smaller than the minimum IOMMU page size,
then we need multiple PAGE_SIZE pages per IOMMU TLB entry. Therefore
those pages must be contiguous. Aside from requiring only hugepage
backing, how could a user make sure that their virtual address buffer
is physically contiguous? I don't think we have any sanity checking
code that does this, thus the warning. Thanks,
Alex
> >> 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@xxxxxxxxxxx>
> >> Signed-off-by: Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx>
> >> ---
> >> 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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