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

Jacob Pan
2021-03-24T22:12:30Z

Hi Jason,

On Wed, 24 Mar 2021 14:03:38 -0300, Jason Gunthorpe <jgg-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org> 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;
}

BTW, IOASID will be one of the resources under the proposed misc cgroup.

Thanks,

Jacob