On Sat, Oct 21, 2023 at 01:38:04PM -0300, Jason Gunthorpe wrote: > On Fri, Oct 20, 2023 at 11:59:13AM -0700, Nicolin Chen wrote: > > 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? > > No, it seems more like tidying that fixing a functional issue, do I > misunderstand? Hmm. Maybe the misunderstanding is mine -- Kevin was asking if it was already a bug and you answered yes: https://lore.kernel.org/linux-iommu/20231016115736.GP3952@xxxxxxxxxx/ If this shouldn't be a bug fix, I could just merge them into a single tidying patch and add the comments you suggested below. > > And where should we add this comment? Kdoc of > > the alloc uAPI? > > Maybe right in front of the only enforce_cc op callback? OK. How about this? Might be a bit verbose though: /* * enforce_cache_coherenc must be determined during the HWPT allocation. * Note that a HWPT (non-CC) created for a device (non-CC) can be later * reused by another device (either non-CC or CC). However, A HWPT (CC) * created for a device (CC) cannot be reused by another device (non-CC) * but only devices (CC). Instead user space in this case would need to * allocate a separate HWPT (non-CC). */ Thanks Nic
