Re: [PATCH 2/2] iommu/sva: Remove mm parameter from SVA bind API

[Lu Baolu]

Hi Jacob,

On 4/14/21 8:09 AM, Jacob Pan wrote:
> Hi Jean,
>
> On Fri, 9 Apr 2021 11:03:05 -0700, Jacob Pan
> <jacob.jun.pan@xxxxxxxxxxxxxxx> wrote:
>
> > > problems:
> > >
> > > * We don't have a use-case for binding the mm of a remote process (and
> > >    it's supposedly difficult for device drivers to do it securely). So
> > > OK, we remove the mm argument from iommu_sva_bind_device() and use the
> > >    current mm. But the IOMMU driver isn't going to do
> > > get_task_mm(current) every time it needs the mm being bound, it will
> > > take it from iommu_sva_bind_device(). Likewise iommu_sva_alloc_pasid()
> > > shouldn't need to bother with get_task_mm().
> > >
> > > * cgroup accounting for IOASIDs needs to be on the current task.
> > > Removing the mm parameter from iommu_sva_alloc_pasid() doesn't help
> > > with that. Sure it indicates that iommu_sva_alloc_pasid() needs a
> > > specific task context but that's only for cgroup purpose, and I'd
> > > rather pass the cgroup down from iommu_sva_bind_device() anyway (but am
> > > fine with keeping it within ioasid_alloc() for now). Plus it's an
> > > internal helper, easy for us to check that the callers are doing the
> > > right thing.
> > With the above split, we really just have one allocation function:
> > ioasid_alloc(), so it can manage current cgroup accounting within. Would
> > this work?
> After a few attempts, I don't think the split can work better. I will
> restore the mm parameter and add a warning if mm != current->mm.

I still worry about supervisor pasid allocation.

If we use iommu_sva_alloc_pasid() to allocate a supervisor pasid, which
mm should the pasid be set? I've ever thought about passing &init_mm to
iommu_sva_alloc_pasid(). But if you add "mm != current->mm", this seems
not to work. Or do you prefer a separated interface for supervisor pasid
allocation/free?

Best regards,
baolu

> Thanks,
>
> Jacob