Re: [PATCH v4 08/17] iommufd: Always setup MSI and anforce cc on kernel-managed domains

[Nicolin Chen]

On Fri, Oct 20, 2023 at 10:55:01AM -0300, Jason Gunthorpe wrote:
> On Fri, Oct 20, 2023 at 02:43:58AM +0000, Tian, Kevin wrote:
> 
> > What we want to prevent is attaching a non-CC device to a CC domain
> > or upgrade a non-CC domain to CC since in both case the non-CC
> > device will be broken due to incompatible page table format.
> 
> [..]
> 
> > Who cares about such consistency? sure the result is different due to order:
> > 
> > 1) creating hwpt for dev1 (non-CC) then later attaching hwpt to
> >     dev2 (CC) will succeed;
> > 
> > 2) creating hwpt for dev2 (CC) then later attaching hwpt to
> >     dev1 (non-CC) will fail then the user should create a new hwpt
> >     for dev1;
> 
> AH... So really what the Intel driver wants is not upgrade to CC but
> *downgrade* from CC.
> 
> non-CC is the type that is universally applicable, so if we come
> across a non-CC capable device the proper/optimal thing is to degrade
> the HWPT and re-use it, not allocate a new HWPT.
> 
> So the whole thing is upside down.
> 
> As changing the IOPTEs in flight seems hard, and I don't want to see
> the Intel driver get slowed down to accomodate this, I think you are
> right to say this should be a creation time property only.
> 
> I still think userspace should be able to select it so it can minimize
> the number of HWPTs required.
> 
> > But the user shouldn't assume such explicit consistency since it's not
> > defined in our uAPI. All we defined is that the attaching may
> > fail due to incompatibility for whatever reason then the user can
> > always try creating a new hwpt for the to-be-attached device. From
> > this regard I don't see providing consistency of result is
> > necessary. 😊
> 
> Anyhow, OK, lets add a comment summarizing your points and remove the
> cc upgrade at attach time (sorry Nicolin/Yi!)

Ack. I will send a small removal series. I assume it should CC
stable tree also? And where should we add this comment? Kdoc of
the alloc uAPI?

Thanks!
Nicolin

> It is easy to add a HWPT flag for this later if someone wants to
> optimize it.
> 
> Jason