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.
