Re: [PATCH v3] iommu: Fix mapping check for 0x0 to avoid re-mapping it

[Jason Gunthorpe <jgg@ziepe.ca>]

On Fri, Feb 27, 2026 at 09:06:37AM +0100, Antheas Kapenekakis wrote:
> Commit 789a5913b29c ("iommu/amd: Use the generic iommu page table")
> introduces the shared iommu page table for AMD IOMMU. Some bioses
> contain an identity mapping for address 0x0, which is not parsed
> properly (e.g., certain Strix Halo devices). This causes the DMA
> components of the device to fail to initialize (e.g., the NVMe SSD
> controller), leading to a failed post.
> 
> Specifically, on the GPD Win 5, the NVME and SSD GPU fail to mount,
> making collecting errors difficult. While debugging, it was found that
> a -EADDRINUSE error was emitted and its source was traced to
> iommu_iova_to_phys(). After adding some debug prints, it was found that
> phys_addr becomes 0, which causes the code to try to re-map the 0
> address and fail, causing a cascade leading to a failed post. This is
> because the GPD Win 5 contains a 0x0-0x1 identity mapping for DMA
> devices, causing it to be repeated for each device.
> 
> The cause of this failure is the following check in
> iommu_create_device_direct_mappings(), where address aliasing is handled
> via the following check:
> 
> ```
> phys_addr = iommu_iova_to_phys(domain, addr);
> if (!phys_addr) {
>         map_size += pg_size;
>         continue;
> }
> ````
> 
> Obviously, the iommu_iova_to_phys() signature is faulty and aliases
> unmapped and 0 together, causing the allocation code to try to
> re-allocate the 0 address per device. However, it has too many
> instantiations to fix. Therefore, use a ternary so that when addr
> is 0, the check is done for address 1 instead.
> 
> Suggested-by: Robin Murphy <robin.murphy@arm.com>
> Fixes: 789a5913b29c ("iommu/amd: Use the generic iommu page table")
> Signed-off-by: Antheas Kapenekakis <lkml@antheas.dev>
> 
> ---

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>

Jason