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

[Jacob Pan <jacob.pan@linux.microsoft.com>]

Hi Jason,

On Mon, 20 Jan 2025 10:26:43 -0400
Jason Gunthorpe <jgg@ziepe.ca> 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