Re: [PATCH v4 3/4] arm-smmu: select suitable MSI IOVA

[Jason Gunthorpe <jgg@ziepe.ca>]

On Tue, Sep 23, 2025 at 08:56:47AM -0700, Shyam Saini wrote:
> Hi Jason, Will,
> 
> On 19 Sep 2025 09:08, Jason Gunthorpe wrote:
> > On Fri, Sep 19, 2025 at 08:33:23AM +0100, Will Deacon wrote:
> > > pieces and will need to work on the userspace side. It's not like
> > > MSI_IOVA2 is magically going to work (and I bet it won't be tested).
> > 
> > It could, if someone checks the default memory map a second constant
> > could be selected that works.
> > 
> > > > Nicolin has some patches on the iommufd side to let userspace select
> > > > the MSI address instead, but they are not done yet.
> > > 
> > > Maybe we should just wait for that? Carrying a temporary hack with ABI
> > > implications to support broken hardware isn't particularly compelling
> > > to me.
> > 
> > This patch would still be needed for kernel users.
> > 
> > Arguably the kernel users should just be using the iova allocator from
> > dma-iommu.c. This whole hard coded constant/sneaky uapi is just a hack
> > to make vfio work..
> > 
> > So maybe if the single constant doesn't work we could set some
> > indication that the caller must allocate the MSI iova, the kernel can
> > use the dma-iommu allocator and VFIO can just refuse to use the device
> > for now.
> 
> So, are we settling on having two predefined MSI IOVA base constants,
> and if both of those conflict with reserved regions on a given platform,
> falling back to dynamic allocation via the IOVA allocator? Just checking
> if that's the consensus we're reaching.

I think Will is arguing against introducing a new constant..

Yesterday I was looking at the SW_MSI code again.. What specific
problem is it you have?

It looks to me like dma-iommu.c is already allocating MSI addresses
using its built in IOVA allocator. So if your DT is marking that space
reserved then it should Just Work right now as dma-iommu.c already
processes the reserved ranges and will allocate MSI addresses around
them?

The base value of the SW_MSI is only used by VFIO - are you trying to
use VFIO with this device, or have I misunderstood the dma-iommu.c
logic?

If it is only VFIO at issue then perhaps we should solve this by
completing the work Nicolin started to allow VFIO userspace to specify
the MSI Aperture?

Jason