Re: [PATCH v3 1/1] vfio/type1: Respect IOMMU reserved regions in vfio_test_domain_fgsp()

[Jason Gunthorpe]

On Tue, Jan 10, 2023 at 05:44:27PM +0100, Niklas Schnelle wrote:
> Since commit cbf7827bc5dc ("iommu/s390: Fix potential s390_domain
> aperture shrinking") the s390 IOMMU driver uses reserved regions for the
> system provided DMA ranges of PCI devices. Previously it reduced the
> size of the IOMMU aperture and checked it on each mapping operation.
> On current machines the system denies use of DMA addresses below 2^32 for
> all PCI devices.
> 
> Usually mapping IOVAs in a reserved regions is harmless until a DMA
> actually tries to utilize the mapping. However on s390 there is
> a virtual PCI device called ISM which is implemented in firmware and
> used for cross LPAR communication. Unlike real PCI devices this device
> does not use the hardware IOMMU but inspects IOMMU translation tables
> directly on IOTLB flush (s390 RPCIT instruction). If it detects IOVA
> mappings outside the allowed ranges it goes into an error state. This
> error state then causes the device to be unavailable to the KVM guest.
> 
> Analysing this we found that vfio_test_domain_fgsp() maps 2 pages at DMA
> address 0 irrespective of the IOMMUs reserved regions. Even if usually
> harmless this seems wrong in the general case so instead go through the
> freshly updated IOVA list and try to find a range that isn't reserved,
> and fits 2 pages, is PAGE_SIZE * 2 aligned. If found use that for
> testing for fine grained super pages.
> 
> Fixes: af029169b8fd ("vfio/type1: Check reserved region conflict and update iova list")
> Signed-off-by: Niklas Schnelle <schnelle@xxxxxxxxxxxxx>
> ---
> v2 -> v3:
> - Don't require region->start to be aligned but instead just that we can fit an
>   aligned allocation (Alex)
> - Use Fixes tag for the introduction of reserved regions as that came
>   after the fine grained super pages test (Alex)
> v1 -> v2:
> - Reworded commit message to hopefully explain things a bit better and
>   highlight that usually just mapping but not issuing DMAs for IOVAs in
>   a resverved region is harmless but still breaks things with ISM devices.
> - Added a check for PAGE_SIZE * 2 alignment (Jason)
> 
>  drivers/vfio/vfio_iommu_type1.c | 31 ++++++++++++++++++++-----------
>  1 file changed, 20 insertions(+), 11 deletions(-)

Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>

Jason