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

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jun 24, 2025 at 04:47:56PM +0100, Jonathan Cameron wrote:

> On x86 there is the much loved WBINVD instruction that causes a write back
> and invalidate of all caches in the system. It is expensive but it is

Expensive is not the only problem. It actively interferes with things
like Cache-Allocation-Technology (RDT-CAT for the intel folks). Doing
WBINVD utterly destroys the cache subsystem for everybody on the
machine.

> necessary in a few corner cases. 

Don't we have things like CLFLUSH/CLFLUSHOPT/CLWB exactly so that we can
avoid doing dumb things like WBINVD ?!?

> These are cases where the contents of
> Physical Memory may change without any writes from the host. Whilst there
> are a few reasons this might happen, the one I care about here is when
> we are adding or removing mappings on CXL. So typically going from
> there being actual memory at a host Physical Address to nothing there
> (reads as zero, writes dropped) or visa-versa. 

> The
> thing that makes it very hard to handle with CPU flushes is that the
> instructions are normally VA based and not guaranteed to reach beyond
> the Point of Coherence or similar. You might be able to (ab)use
> various flush operations intended to ensure persistence memory but
> in general they don't work either.

Urgh so this. Dan, Dave, are we getting new instructions to deal with
this? I'm really not keen on having WBINVD in active use.