Re: [PATCH v10 08/12] iommu/sva: Refactoring iommu_sva_bind/unbind_device()

[Jason Gunthorpe]

On Mon, Jul 25, 2022 at 06:22:06PM +0800, Baolu Lu wrote:
> On 2022/7/25 15:50, Tian, Kevin wrote:
> > > From: Baolu Lu <baolu.lu@xxxxxxxxxxxxxxx>
> > > Sent: Sunday, July 24, 2022 9:48 PM
> > > > 
> > > > The API is really refcounting the PASID:
> > > > 
> > > > > +struct iommu_sva *iommu_sva_bind_device(struct device *dev,
> > > > > +					struct mm_struct *mm);
> > > > > +void iommu_sva_unbind_device(struct iommu_sva *handle);
> > > > 
> > > > So what you need to do is store that 'iommu_sva' in the group's PASID
> > > > xarray.
> > > > 
> > > > The bind logic would be
> > > > 
> > > >     sva = xa_load(group->pasid, mm->pasid)
> > > >     if (sva)
> > > >        refcount_inc(sva->users)
> > > >        return sva
> > > >     sva = kalloc
> > > >     sva->domain = domain
> > > >     xa_store(group->pasid, sva);
> > > 
> > > Thanks for the suggestion. It makes a lot of sense to me.
> > > 
> > > Furthermore, I'd like to separate the generic data from the caller-
> > > specific things because the group->pasid_array should also be able to
> > > serve other usages. Hence, the attach/detach_device_pasid interfaces
> > > might be changed like below:
> > > 
> > > /* Collection of per-pasid IOMMU data */
> > > struct group_pasid {
> > > 	struct iommu_domain *domain;
> > > 	void *priv;
> > > };
> > > 
> > 
> > Is there any reason why pasid refcnt is sva specific and needs to be
> > in a priv field?
> 
> I am going to store the iommu_sva data which represents the bind
> relationship between device and domain.

Why do you need that?

If you are starting at the pasid xarray then you already know the
group/device, so we don't need to store it again.

The only thing needed is the refcount so just store a refcount in this
structure and be done with it. If someone needs to add something later
then we can use a union or something, but right now adding an untagged
void * is bad.

Jason