Re: [PATCH 1/4] ioasid: Reserve a global PASID for in-kernel DMA

[Jacob Pan]

Hi Jason,

On Fri, 10 Dec 2021 08:31:09 -0400, Jason Gunthorpe <jgg@xxxxxxxxxx> wrote:

> On Fri, Dec 10, 2021 at 09:06:24AM +0000, Jean-Philippe Brucker wrote:
> > On Thu, Dec 09, 2021 at 10:14:04AM -0800, Jacob Pan wrote:  
> > > > This looks like we're just one step away from device drivers needing
> > > > multiple PASIDs for kernel DMA so I'm trying to figure out how to
> > > > evolve the API towards that. It's probably as simple as keeping a
> > > > kernel IOASID set at first, but then we'll probably want to
> > > > optimize by having multiple overlapping sets for each device driver
> > > > (all separate from the SVA set).  
> > > Sounds reasonable to start with a kernel set for in-kernel DMA once
> > > we need multiple ones. But I am not sure what *overlapping* sets mean
> > > here, could you explain?  
> > 
> > Given that each device uses a separate PASID table, we could allocate
> > the same set of PASID values for different device drivers. We just need
> > to make sure that those values are different from PASIDs allocated for
> > user SVA.  
> 
> Why does user SVA need global values anyhow?
> 
Currently, we have mm.pasid for user SVA. mm is global. We could have per
device PASID for dedicated devices (not shared across mm's), but that would
make things a lot more complex. I am thinking multiple PASIDs per mm is
needed, right?

For VT-d, the shared workqueue (SWQ) requires global PASIDs in that we
cannot have two processes use the same PASID to submit work on a workqueue
shared by the two processes. Each process's PASID must be unique to the
SWQ's PASID table.

> Jason


Thanks,

Jacob