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

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Date:   Tue, 29 Nov 2022 16:09:11 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Niklas Schnelle <schnelle@...ux.ibm.com>,
        Baolu Lu <baolu.lu@...ux.intel.com>,
        Matthew Rosato <mjrosato@...ux.ibm.com>,
        Gerd Bayer <gbayer@...ux.ibm.com>, iommu@...ts.linux.dev,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Wenjia Zhang <wenjia@...ux.ibm.com>,
        Pierre Morel <pmorel@...ux.ibm.com>,
        linux-s390@...r.kernel.org, borntraeger@...ux.ibm.com,
        hca@...ux.ibm.com, gor@...ux.ibm.com,
        gerald.schaefer@...ux.ibm.com, agordeev@...ux.ibm.com,
        svens@...ux.ibm.com, linux-kernel@...r.kernel.org,
        Julian Ruess <julianr@...ux.ibm.com>
Subject: Re: [PATCH v2 4/7] iommu: Let iommu.strict override
 ops->def_domain_type

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.

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