Re: [PATCH v5 3/6] iommu/s390: Fix potential s390_domain aperture shrinking

[Niklas Schnelle <schnelle@linux.ibm.com>]

On Thu, 2022-10-06 at 16:46 +0200, Niklas Schnelle wrote:
> The s390 IOMMU driver currently sets the IOMMU domain's aperture to
> match the device specific DMA address range of the device that is first
> attached. This is not ideal. For one if the domain has no device
> attached in the meantime the aperture could be shrunk allowing
> translations outside the aperture to exist in the translation tables.
> Also this is a bit of a misuse of the aperture which really should
> describe what addresses can be translated and not some device specific
> limitations.
> 
> Instead of misusing the aperture like this we can instead create
> reserved ranges for the ranges inaccessible to the attached devices
> allowing devices with overlapping ranges to still share an IOMMU domain.
> This also significantly simplifies s390_iommu_attach_device() allowing
> us to move the aperture check to the beginning of the function and
> removing the need to hold the device list's lock to check the aperture.
> 
> As we then use the same aperture for all domains and it only depends on
> the table properties we can already check zdev->start_dma/end_dma at
> probe time and turn the check on attach into a WARN_ON().
> 
> Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
> Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>

@Matt, @Jason I did drop the R-b's here because the change Jason
suggested of changing the aperture check on attach to a WARN_ON() and
checking zdev->start_dma/end_dma on probe is a behavioral change.

> ---
> v4->v5:
> - Make aperture check in attach a WARN_ON() and fail in probe if
>   zdev->start_dma/end_dma doesn't git in aperture  (Jason)
> 
>  drivers/iommu/s390-iommu.c | 65 +++++++++++++++++++++++++-------------
>  1 file changed, 43 insertions(+), 22 deletions(-)
> 
> diff --git a/drivers/iommu/s390-iommu.c b/drivers/iommu/s390-iommu.c
> index 9b3ae4b14636..1f6c9bee9a80 100644
> --- a/drivers/iommu/s390-iommu.c
---8<---