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
