Re: [PATCH v2 4/7] iommu: Let iommu.strict override ops->def_domain_type

[Niklas Schnelle <schnelle@linux.ibm.com>]

On Tue, 2022-11-29 at 16:09 -0400, Jason Gunthorpe wrote:
> On Tue, Nov 29, 2022 at 06:41:22PM +0000, Robin Murphy wrote:
> > On 2022-11-29 17:33, Jason Gunthorpe wrote:
> > > On Mon, Nov 28, 2022 at 09:01:43PM +0000, Robin Murphy wrote:
> > > 
> > > > I'm hardly an advocate for trying to save users from themselves, but I
> > > > honestly can't see any justifiable reason for not having sysfs respect
> > > > iommu_get_def_domain_type().
> > > 
> > > We really need to rename this value if it is not actually just an
> > > advisory "default" but a functional requirement ..
> > 
> > It represents a required default domain type. As in, the type for the
> > device's default domain. Not the default type for a domain. It's the
> > iommu_def_domain_type variable that holds the *default* default domain type
> > ;)
> 
> I find the name "default domain" incredibly confusing at this point in
> time.

Yes it definitely confused me as evidenced by this patch.

> 
> I would like to call that the "dma-api domain" - its primary purpose
> is to be the domain that the DMA API uses to operate the IOMMU, there
> is little "default" about it. This meshes better with our apis talking
> about ownership and so forth.
> 
> So, if the op was called
>   get_dma_api_domain_type()
> 
> It is pretty clear that it is the exact type of domain that should be
> created to support the DMA API, which is what I think you have been
> describing it is supposed to do?
> 
> And with Lu's series we have the set_platform_dma() (Lu perhaps you
> should call this set_platform_dma_api() to re-enforce it is about the
> DMA API, not some nebulous DMA thing)
> 
> Which is basically the other way to configure the DMA API for
> operation.
> 
> And encapsulating more of the logic to setup and manage the DMA API's
> domain into dma-iommu.c would also be helpful to understanding.
> 
> > Which reminds me I should finish that patch undoing my terrible
> > ops->default_domain_ops idea, not least because they are misleadingly
> > unrelated to default domains...
> 
> :)
> 
> > > It is close to being clear, once we get the last touches of dma-iommu
> > > stuff out of the drivers it should be quite clear
> > 
> > Cool, some upheaval of .domain_alloc is next on my hitlist anyway, so that
> > might be a good excuse to upheave it a bit more and streamline the type
> > stuff along the way.
> 
> Yes, I think so. I want to tidy things a bit so adding this "user
> space" domain concept is a little nicer
> 
> Jason

Ok, so it sounds to me like there is going to be quite a bit of change
in this area. Thus I'm a little unsure however how to proceed here. I
think especially with the proposed multiple queue types in this series
it makes sense for an IOMMU driver to express its preference of a
particular domain type if it supports multiple which clearly isn't what
iommu_get_def_domain_type() is currently supposed to do.

Looking at the current code I don't see a trivial way to integrate this
especially with a lot of reworks going on.

At the moment the preference for IOMMU_DOMAIN_DMA_FQ over
IOMMU_DOMAIN_DMA during initial boot is hard coded in
iommu_subsys_init() before we even know what IOMMU driver will be used.
so can't really access its ops there. On the other hand this decision
may still be rolled back if iommu_dma_init_fq() fails in
iommu_dma_init_domain() so maybe it would make sense to assume a DMA
domain type of IOMMU_DOMAIN_DMA until that point and only then prefer
IOMMU_DOMAIN_DMA_FQ or something else if the driver has a preference.

Maybe it would also make sense not to mix the type of flush queue used
with the domain and maybe the queue type could be passed in via
iommu_setup_dma_ops() -> iommu_dma_init_domain() though I do remember
that there is also a planned rework for that. Any suggestions?

Thanks,
Niklas