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

[Jacob Pan]

Hi Jason,

On Thu, 25 Mar 2021 14:16:45 -0300, Jason Gunthorpe <jgg@xxxxxxxxxx> wrote:

> On Thu, Mar 25, 2021 at 10:02:36AM -0700, Jacob Pan wrote:
> > Hi Jean-Philippe,
> > 
> > On Thu, 25 Mar 2021 11:21:40 +0100, Jean-Philippe Brucker
> > <jean-philippe@xxxxxxxxxx> wrote:
> >   
> > > On Wed, Mar 24, 2021 at 03:12:30PM -0700, Jacob Pan wrote:  
> > > > Hi Jason,
> > > > 
> > > > On Wed, 24 Mar 2021 14:03:38 -0300, Jason Gunthorpe <jgg@xxxxxxxxxx>
> > > > wrote:   
> > > > > On Wed, Mar 24, 2021 at 10:02:46AM -0700, Jacob Pan wrote:    
> > > > > > > Also wondering about device driver allocating auxiliary
> > > > > > > domains for their private use, to do iommu_map/unmap on
> > > > > > > private PASIDs (a clean replacement to super SVA, for
> > > > > > > example). Would that go through the same path as /dev/ioasid
> > > > > > > and use the cgroup of current task?      
> > > > > >
> > > > > > For the in-kernel private use, I don't think we should restrict
> > > > > > based on cgroup, since there is no affinity to user processes. I
> > > > > > also think the PASID allocation should just use kernel API
> > > > > > instead of /dev/ioasid. Why would user space need to know the
> > > > > > actual PASID # for device private domains? Maybe I missed your
> > > > > > idea?      
> > > > > 
> > > > > There is not much in the kernel that isn't triggered by a
> > > > > process, I would be careful about the idea that there is a class
> > > > > of users that can consume a cgroup controlled resource without
> > > > > being inside the cgroup.
> > > > > 
> > > > > We've got into trouble before overlooking this and with something
> > > > > greenfield like PASID it would be best built in to the API to
> > > > > prevent a mistake. eg accepting a cgroup or process input to the
> > > > > allocator. 
> > > > Make sense. But I think we only allow charging the current cgroup,
> > > > how about I add the following to ioasid_alloc():
> > > > 
> > > > 	misc_cg = get_current_misc_cg();
> > > > 	ret = misc_cg_try_charge(MISC_CG_RES_IOASID, misc_cg, 1);
> > > > 	if (ret) {
> > > > 		put_misc_cg(misc_cg);
> > > > 		return ret;
> > > > 	}    
> > > 
> > > Does that allow PASID allocation during driver probe, in kernel_init
> > > or modprobe context?
> > >   
> > Good point. Yes, you can get cgroup subsystem state in kernel_init for
> > charging/uncharging. I would think module_init should work also since
> > it is after kernel_init. I have tried the following:
> > static int __ref kernel_init(void *unused)
> >  {
> >         int ret;
> > +       struct cgroup_subsys_state *css;
> > +       css = task_get_css(current, pids_cgrp_id);
> > 
> > But that would imply:
> > 1. IOASID has to be built-in, not as module
> > 2. IOASIDs charged on PID1/init would not subject to cgroup limit since
> > it will be in the root cgroup and we don't support migration nor will
> > migrate.
> > 
> > Then it comes back to the question of why do we try to limit in-kernel
> > users per cgroup if we can't enforce these cases.  
> 
> Are these real use cases? Why would a driver binding to a device
> create a single kernel pasid at bind time? Why wouldn't it use
> untagged DMA?
> 
For VT-d, I don't see such use cases. All PASID allocations by the kernel
drivers has proper process context.

> When someone needs it they can rework it and explain why they are
> doing something sane.
> 
Agreed.

> Jason


Thanks,

Jacob