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

On Wed, Jun 25, 2025 at 02:12:39AM -0700, H. Peter Anvin wrote:

On June 25, 2025 1:52:04 AM PDT, Peter Zijlstra [off-list ref] wrote:

quoted

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

quoted

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.

quoted

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 ?!?

quoted

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. 

quoted

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.

Right this... But that CXL thing sounds like that's semi 'regular' to
the point that providing some infrastructure around it makes sense. This
should not be.