Re: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and allocation APIs

[Jean-Philippe Brucker]

On Tue, Mar 30, 2021 at 10:07:55AM -0300, Jason Gunthorpe wrote:
> On Fri, Mar 26, 2021 at 09:06:42AM +0100, Jean-Philippe Brucker wrote:
> 
> > It's not inconceivable to have a control queue doing DMA tagged with
> > PASID. The devices I know either use untagged DMA, or have a choice to use
> > a PASID.
> 
> I don't think we should encourage that. A PASID and all the related is
> so expensive compared to just doing normal untagged kernel DMA.

How is it expensive?  Low number of PASIDs, or slowing down DMA
transactions?  PASIDs aren't a scarce resource on Arm systems, they have
almost 1M unused PASIDs per VM.

Thanks,
Jean

> I assume HW has these features because virtualization use cases might
> use them, eg by using mdev to assign a command queue - then it would
> need be be contained by a PASID.
> 
> Jason