Re: [PATCH v2 0/8] Cache coherency management subsystem

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Jul 09, 2025 at 10:22:40PM -0700, dan.j.williams@intel.com wrote:

> "Regular?", no. Something is wrong if you are doing this regularly. In
> current CXL systems the expectation is to suffer a WBINVD event once per
> server provisioning event.

Ok, so how about we strictly track this once, and when it happens more
than this once, we error out hard?

> Now, there is a nascent capability called "Dynamic Capacity Devices"
> (DCD) where the CXL configuration is able to change at runtime with
> multiple hosts sharing a pool of memory. Each time the physical memory
> capacity changes, cache management is needed.
> 
> For DCD, I think the negative effects of WBINVD are a *useful* stick to
> move device vendors to stop relying on software to solve this problem.
> They can implement an existing CXL protocol where the device tells CPUs
> and other CXL.cache agents to invalidate the physical address ranges
> that the device owns.
> 
> In other words, if WBINVD makes DCD inviable that is a useful outcome
> because it motivates unburdening Linux long term with this problem.

Per the above, I suggest we not support this feature *AT*ALL* until an
alternative to WBINVD is provided.

> In the near term though, current CXL platforms that do not support
> device-initiated-invalidate still need coarse cache management for that
> original infrequent provisioning events. Folks that want to go further
> and attempt frequent DCD events with WBINVD get to keep all the pieces.

I would strongly prefer those pieces to include WARNs and or worse.