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

[Jason Gunthorpe]

On Thu, Sep 18, 2025 at 05:49:39PM +0100, Will Deacon wrote:
> Given that we're walking over the reserved regions to see if we have a
> collision with MSI_IOVA_BASE, why not allocate the base address
> dynamically if we detect a collision rather than having yet another
> hard-coded address which we can't guarantee won't be problematic in future?

I'm nervous about this. Right now the  MSI_IOVA is actually UAPI and
things like VFIO/qmeu need to accomodate it's placement in their own
memory maps.

Nicolin has some patches on the iommufd side to let userspace select
the MSI address instead, but they are not done yet.

So, randomly picking an address sounds like a bad idea to me, it would
almost certainly unpredictably break qemu..

Hopefully by the time we need a 3rd one we have the userspace MSI
control and then we could make reasonably make the kernel
automatically select.

Jason