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
