Re: [PATCH 0/3] make MSI IOVA base address and its length configurable

[Jason Gunthorpe <jgg@ziepe.ca>]

On Thu, Jan 16, 2025 at 03:23:04PM -0800, Shyam Saini wrote:
> Hi,
> 
> Currently, the MSI_IOVA_BASE address is hard-coded to 0x80000000,
> assuming that all platforms have this address available for MSI IOVA
> reservation. However, this is not always the case, as some platforms
> reserve this address for other purposes.

Can you explain this some more? This address is in the kernel
controlled IOVA space, there are few ways a platform can impact this.

How is the platform impacting it? Is the non-functional IOVA always
reflected in the iommu_get_resv_regions()?

Why not avoid this conflict in your platform software?

> There was an [1] attempt to fix this problem by passing the MSI IOVA
> base as a kernel command line parameter. 

Yuk

> In the previous attempt,
> Will suggested reserving the MSI IOVA at runtime whenever there is a
> conflict with the default MSI_IOVA_BASE. However, dynamically reserving
> this address has debuggability concerns, as it becomes difficult to
> track IOMMU mapping failures.

Still, this approach seems like the best to me..

> This patch series aims to address the issue by introducing a new DTS
> property, "arm,smmu-pci-msi-iova-data". This property allows the
> configuration of MSI IOVA with a custom MSI base address and a custom
> length for IOMMU/SMMU drivers. It accommodates platforms that do not
> have the default MSI base address available for MSI reservation.

My understand was using DT to set kernel configurables was frowned
upon? Ultimately MSI_IOVA_BASE is an arbitary choice by kernel
software.

Jason