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

[Will Deacon <will@kernel.org>]

On Thu, Sep 18, 2025 at 07:43:22PM -0300, Jason Gunthorpe wrote:
> 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.

*shrug*

That's only the case for broken systems where the existing MSI_IOVA
can't be made to work. As far as I'm concerned, they get to keep the
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).

> 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.

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

It's not really "random", in the sense that it would be deterministic
for a given set of reserved regions.

Will