Re: Preferred method to detect if a device is behind an enabled iommu.

[Robin Murphy <robin.murphy-5wv7dgnIgG8@public.gmane.org>]

On 01/02/18 10:18, Jonathan Cameron wrote:
> Hi All,
> 
> We have a crypto accelerator which needs to have a few different settings
> depending on whether or not the SMMUv3 is enabled and translating addresses
> or not.
> 
> https://marc.info/?l=linux-crypto-vger&m=151732626428206&w=2
> 
> 1) A quirk of the hardware revision means we need to turn some elements
>     off if the iommu is enabled.
> 2) The device has certain cache related settings that means it needs to know
>     if it is dealing with VAs or PAs.
> 
> Current approach is to see if the iommu_group is set in struct device.
> 
> We could fine one instance of another driver doing this and copied that,
> (drivers/dma/rcar-dmac.c)
> but the precedence is weak enough that confirmation would be good.
> So whilst it 'works' the question is whether it is safe in general
> and whether there is a better way.

The presence of a group alone is not sufficient, as it only tells you 
that the device is associated with an IOMMU in some way (including VFIO 
no-iommu mode where said IOMMU isn't even real).

To detect whether translation is active, I think the best way right now 
would be to first call iommu_get_domain_for_dev() to see whether the 
device is actually attached to a domain, then if so check the domain 
type for the __IOMMU_DOMAIN_PAGING flag to confirm if it represents a 
translation context rather than a bypass one.

It might be reasonable to propose wrapping that up in an IOMMU API (or 
possibly DMA API, as appropriate) helper, as there are certainly other 
drivers doing various degrees of this sort of thing for various reasons 
(to the point where we currently have to accommodate rather nonsensical 
iova_to_phys() calls on identity domains).

Robin.