Device specific pass through in host systems - discuss user interface

[Raj, Ashok]

Hi Sai

On Sun, Jun 09, 2019 at 10:41:10PM -0700, Sai Praneeth Prakhya wrote:
> > > I am working on an IOMMU driver feature that allows a user to specify
> > > if the DMA from a device should be translated by IOMMU or not.
> > > Presently, we support only all devices or none mode i.e. if user
> > > specifies "iommu=pt" [X86] or "iommu.passthrough" [ARM64] through
> > > kernel command line, all the devices would be in pass through mode and
> > > we don't have per device granularity, but, we were requested by a
> > > customer to selectively put devices in pass through mode and not all.
> > 
> > Most iommu vendor drivers have switched from per-device to per-group domain
> > (a.k.a. default domain). So per-group pass-through mode makes more sense?
> > 
> > By the way, can we extend this to "per-group default domain type", instead of
> > only "per-group pass-through mode"? Currently we have system level default
> > domain type, if we have finer granularity of default domain type, both iommu
> > drivers and end users will benefit from it.
> 
> Sure! Makes sense.. per-group default domain type sounds good.
> 
> > > I am looking for a consensus on **how the kernel command line argument
> > > should look like and path for sysfs entry**. Also, please note that if
> > > a device is put in pass through mode it won't be available for the
> > > guest and that's ok.
> > 
> > Just out of curiosity, what's the limitation for a device using pass- through DMA
> > domain to be assignable.
> 
> Sorry! I don't know about assignable devices. Probably, Ashok or Jacob could answer this question

We don't switch the domain for assigned devices. Only the "type" of the default domain is 
changed from dma-protected to passthrough type.

When assigning devices to user-space, there is no change in this proposal.

> 
> Regards,
> Sai