[PATCH v2 0/3] arm-smmu: select suitable IOVA

[Jason Gunthorpe]

On Thu, May 29, 2025 at 11:22:19AM -0700, Shyam Saini wrote:
> > All IOVA that the platform cannot DMA from should be reported in the
> > reserved_regions file as "reserved". You must make your platform
> > achieve this.
> 
> so should it be for all the iommu groups?
> 
>                 no_dma_mem {
>                        reg = <0x0 0x8000000 0x0 0x100000>;
>                         no-map;
>                 };
>  
> i think that's how we reserve memory in general, eg: ramoops
> but this doesn't show up in:
>   /sys/kernel/iommu_groups/*/reserved_regions

I don't know anymore, I've forgotten these details about DT, you will
need to consult with DT experts..

> > But I still don't have a clear sense of what your actual problem is as
> > you are show DT that seems reasonable and saying that
> > /sys/../reserved_regions is working..
> 
> /sys/../reserved_regions is working for certain devices like dma controller
> but it doesn't work for pcie devices and its vfio-pcie driver calling
> iommu_get_resv_regions() but we don't have dts node for vfio.
> I have confirmed this about pcie on two different platforms, it seems to be
> OF DMA-API gap that you hinted above, happy to work on that :), it would be
> great if you can share any other reference discussions to that problem

Some of it was around the dma_mask and the ranges, but I'm not sure
exactly what is your issue here..

It is very important the the dma-iommu.c code also knows not to use
the non-working IOVA or your system will be buggy and eventually fail.
If that isn't happening you have a critical bug beyond the SW_MSI
issue.

The easiest way to fix it is to have reserved_regions report the right
stuff.

> When i specify this for dma controller:
> 
> 		faulty_iova: resv_faulty {
> 			iommu-addresses = <&dmaX 0x8000000 0x100000>;
> 		};
> &dmaX {
> 	memory-region = <&faulty_iova>;
> };
> 
> I see following:
> $ cat /sys/kernel/iommu_groups/y/reserved_regions 
> 0x0000000008000000 0x00000000080fffff reserved
> 0x00000000a0000000 0x00000000a00fffff msi

See, that looks better. You need to make that happen for all the
effected devices.

> Upon investigation, our hardware team confirmed that the memory region
> containing 0x08000000 is already mapped for other peripherals, making it
> unavailable for MSI usage.

See, that's just wrong HW design. The path from PCI to the SMMU should
not have any other decodes on it, isn't that SBSA?

How does your platform work at all? Isn't 0x08000000 physical memory in
your address map?

Jason