Hi Jason, On Mon, 20 Jan 2025 10:26:43 -0400 Jason Gunthorpe <jgg@xxxxxxxx> wrote: > 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()? I don't know the platform impact but just to clarify, are you asking whether this non-functional IOVA is also under IORT RMR or other FW tables? I don't think it is. But this special IOVA is reflected in iommu_get_resv_regions() the same way as the hardcoded MSI_IOVA_BASE. So each iommu group's reserved_regions should show. > Why not avoid this conflict in your platform software? I had the same question but it seems there is not enough difference (than the standard smmu) to justify a platform code. i.e. platform specific iommu_get_resv_regions(), is that what you are suggesting? > > 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
