On Wed, 25 Jun 2025, H. Peter Anvin wrote:
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.
Correct, and cpu_cache_invalidate_memregion() was introduced exactly
with these constraints in mind, and the current x86 is the worse case
scenario. As Jonathan pointed out, ranged optimizations only improve
what is already there.
Thanks,
Davidlohr
