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

[Antheas Kapenekakis]

On Fri, 27 Feb 2026 at 15:03, Jason Gunthorpe <jgg@xxxxxxxx> wrote:
>
> 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@xxxxxxx>
> > Fixes: 789a5913b29c ("iommu/amd: Use the generic iommu page table")
> > Signed-off-by: Antheas Kapenekakis <lkml@xxxxxxxxxxx>
> >
> > ---
>
> Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>

Tested. It should be good to merge

Antheas

> Jason
>