Messages in this thread | | | Date | Fri, 20 Oct 2023 10:55:01 -0300 | From | Jason Gunthorpe <> | Subject | Re: [PATCH v4 08/17] iommufd: Always setup MSI and anforce cc on kernel-managed domains |
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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!)
It is easy to add a HWPT flag for this later if someone wants to optimize it.
Jason
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