[PATCH v2 4/4] iommu/arm: Add BBM Level 2 smmu feature

[Jason Gunthorpe]

On Mon, Mar 03, 2025 at 10:31:02AM +0000, Mikołaj Lenczewski wrote:

> > > On such a system it seems like your series would break previously
> > > working SVA support because this patch will end up disabling it?
> 
> Perhaps my understanding is flawed here, but I was under the impression
> that with SVA both the core and smmu MUST support BBML2 to use it safely
> for core translations? 

Yes

But today's kernel does not use BBML2 in the CPU or the SMMU so it is
compatible with everything.

So it is an upgrade issue, going from today's kernel without any BBML2
support to tomorrow's kernel that does then you loose SVA on
previously working HW.

> Hopefully, as you say, the MIDR list restricts the breakage to a limited
> (ideally, zero-size) set of implementations which advertise BBML2
> without conflict aborts, but which do not support BBML2 on the smmu.
> 
> However, if my understanding of the BBML2 feature and how it interacts
> with SVA is flawed, this will obviously be something for me to fix.

Lets hope, I was not able to discover any NVIDIA platforms that have
an issue with this series as is.

But every addition to the MIDR list will require some consideration :\

> On independently enabling BBML2 on the smmu but not the CPU, this should
> be possible.

What about the reverse? Could we disable BBML2 on the CPU side on a
per-mm basis? Ie when an old SMMU attaches with disable the
incompatible feature? Not something for this series, but if we get
into trouble down the road

Jason