Re: [PATCH 0/5] Share sva domains with all devices bound to a mm

[Tina Zhang]

Hi,

On 8/9/23 18:51, Baolu Lu wrote:
> On 2023/8/9 17:44, Tian, Kevin wrote:
> > > From: Baolu Lu<baolu.lu@xxxxxxxxxxxxxxx>
> > > Sent: Wednesday, August 9, 2023 8:18 AM
> > >
> > > On 2023/8/8 15:49, Tina Zhang wrote:
> > > > A sva domain's lifetime begins with binding a device to a mm and ends
> > > > by releasing all the bound devices from that sva domain. Technically,
> > > > there could be more than one sva domain identified by the mm PASID for
> > > > the use of bound devices issuing DMA transactions.
> > > >
> > > > To support mm PASID 1:n with sva domains, each mm needs to keep both
> > > a
> > > > reference list of allocated sva domains and the corresponding PASID.
> > > > However, currently, mm struct only has one pasid field for sva usage,
> > > > which is used to keep the info of an assigned PASID. That pasid field
> > > > cannot provide sufficient info to build up the 1:n mapping between 
> > > > PASID
> > > > and sva domains.
> > > Is it more appropriate to have the same life cycle for sva domain and mm
> > > pasid? I feel that they represent the same thing, that is, the address
> > > space shared by mm to a device.
> > iirc it's a simplification to free mm pasid at __mmdrop() otherwise the
> > implementation is tricky, but I don't remember all the detail...
>
> Yeah, probably we could also free the sva domains in __mmdrop()? Remove
> the refcount for sva domain just like what we did for pasid (at the
> beginning we had refcount for each pasid...).

For sva usage, mm->mm_count is increased in iommu_sva_domain_alloc(), 
and gets decreased when the domain has no users (which is checked in 
iommu_sva_unbind_device()).

So, in a mm's life time, there could be multiple sva domains, though 
they are using the same PASID. I think it makes sense to mm. Because it 
makes no sense to keep a sva domain alive when no users are using it, 
even though the mm is alive.

Regards,
-Tina

> Best regards,
> baolu