Re: [PATCH v4 05/11] iommu/sva: Assign a PASID to mm on PASID allocation and free it on mm exit

Luck, Tony
2022-02-10T17:25:00Z

On Thu, Feb 10, 2022 at 08:27:50AM -0800, Fenghua Yu wrote:
> Hi, Jacob,
>
> On Wed, Feb 09, 2022 at 07:16:14PM -0800, Jacob Pan wrote:
> > Hi Fenghua,
> >
> > On Mon, 7 Feb 2022 15:02:48 -0800, Fenghua Yu <fenghua.yu@intel.com> wrote:
> >
> > > @@ -1047,8 +1040,6 @@ struct iommu_sva *intel_svm_bind(struct device
> > > *dev, struct mm_struct *mm, void }
> > >
> > > sva = intel_svm_bind_mm(iommu, dev, mm, flags);
> > > - if (IS_ERR_OR_NULL(sva))
> > > - intel_svm_free_pasid(mm);
> > If bind fails, the PASID has no IOMMU nor CPU context. It should be safe to
> > free here.
>
> The PASID can not be freed even if bind fails. The PASID allocated earlier
> (either in this thread or in another thread) might be populated to other
> threads already and being used now.
>
> Without freeing the PASID on bind failure, the worst case is the PASID might
> not be used in the process (and will be freed on process exit anyway).
>
> This all matches with the PASID life time described in the commit message.

But what does this mean for the user that failed that intel_svm_bind_mm()?

Here's a scenario:

Process sets up to use PASID capable device #1. Everything works,
so the process has mm->pasid, and the IOMMU has the tables to map
virtual addresses coming from device #1 using that PASID.

Now the same process asks to start using PASID capable device #2,
but there is a failure at intel_svm_bind_mm().

Fenghua is right that we shouldn't free the PASID. It is in use
by at least one thread of the process to access device #1.

But what happens with device #2? Does the caller of intel_svm_bind()
do the right thing with the IS_ERR_OR_NULL return value to let the
user know that device #2 isn't accessible?

-Tony