Re: [PATCH v3 1/4] iommu: Introduce device_iommu_capable()

[Robin Murphy]

On 2022-04-06 06:28, Christoph Hellwig wrote:
> On Tue, Apr 05, 2022 at 11:41:01AM +0100, Robin Murphy wrote:
> > iommu_capable() only really works for systems where all IOMMU instances
> > are completely homogeneous, and all devices are IOMMU-mapped. Implement
> > the new variant which can give an accurate answer for whichever device
> > the caller is actually interested in.
> >
> > Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx>
> > ---
> >
> > v3: New patch; now that the dev_iommu_ops() work has landed we can go
> >      straight to a proper implementation. Also s/dev/device/ to match
> >      the precedent of device_iommu_mapped() for the public API.
>
> I'm a little worrited about a method with a parameter than can be
> NULL.

FWIW, the intent is that that's only temporary.

> Also usnic, vmd, and vdpa really want to use your new
> device_iommu_capable as they check based on a device.  Just VFIO
> is special as usual..

Indeed, I have those patches in my stack already, I'm just waiting for 
this to land before I think about posting them. Once the DMA ownership 
series lands in parallel, VFIO can also get converted and 
iommu_capable() goes away entirely. At that point I have a patch for the 
Arm SMMU drivers to start actually using the new device argument, but 
Jason's new proposal now puts a twist on that...

On reflection, there's no real need for the internal method to change in 
lock-step with the external interface. I've no objection to holding off 
on adding that new parameter until it's reliably useful, and indeed now 
that I'm looking at it outside the context of the entire cleanup 
mission, that does sound like the right thing to do anyway :)

(also it turns out I need to respin this patch regardless since I 
generated it from the wrong point in my branch, where iommu_present() 
was already gone from the context in iommu.h...)

Cheers,
Robin.