On June 25, 2025 1:52:04 AM PDT, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: >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. > WBINVD is the nuclear weapon to use when you have lost all notion of where the problematic data can be, and amounts to a full reset of the cache system. WBINVD can block interrupts for many *milliseconds*, system wide, and so is really only useful for once-per-boot type events, like MTRR initialization.
