Re: [RFC PATCH 0/4] iommu: Per-group default domain type

[John Garry]

On 21/01/2020 00:43, Lu Baolu wrote:
> > > An IOMMU group represents the smallest set of devices that are 
> > > considered
> > > to be isolated. All devices belonging to an IOMMU group share a default
> > > domain for DMA APIs. There are two types of default domain: 
> > > IOMMU_DOMAIN_DMA
> > > and IOMMU_DOMAIN_IDENTITY. The former means IOMMU translation, while the
> > > latter means IOMMU by-pass.
> > >
> > > Currently, the default domain type for the IOMMU groups is determined
> > > globally. All IOMMU groups use a single default domain type. The global
> > > default domain type can be adjusted by kernel build configuration or
> > > kernel parameters.
> > >
> > > More and more users are looking forward to a fine grained default domain
> > > type. For example, with the global default domain type set to 
> > > translation,
> > > the OEM verndors or end users might want some trusted and fast-speed 
> > > devices
> > > to bypass IOMMU for performance gains. On the other hand, with global
> > > default domain type set to by-pass, some devices with limited system
> > > memory addressing capability might want IOMMU translation to remove the
> > > bounce buffer overhead.
> >
> > Hi Lu Baolu,
> >
> > Do you think that it would be a more common usecase to want 
> > kernel-managed devices to be passthrough for performance reasons and 
> > some select devices to be in DMA domain, like those with limited 
> > address cap or whose drivers request huge amounts of memory?
> >
> > I just think it would be more manageable to set kernel commandline 
> > parameters for this, i.e. those select few which want DMA domain.

Hi Baolu,

> It's just two sides of a coin. Currently, iommu subsystem make DMA
> domain by default, that's the reason why I selected to let user set
> which devices are willing to use identity domains.

OK, understood.

There was an alternate solution here which would allow per-group type to 
be updated via sysfs:

https://lore.kernel.org/linux-iommu/cover.1566353521.git.sai.praneeth.prakhya@xxxxxxxxx/

Any idea what happened to that?

Cheers,
John