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

[Shyam Saini]

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.

Thanks,
Shyam