Re: [RFC PATCH 0/6] Cache coherency management subsystem

Jonathan Cameron
2025-03-24T12:03:03Z

On Fri, 21 Mar 2025 22:32:15 +0000
Conor Dooley <conor@kernel.org> wrote:

> On Thu, Mar 20, 2025 at 05:41:12PM +0000, Jonathan Cameron wrote:
> > Note that I've only a vague idea of who will care about this
> > so please do +CC others as needed.
> >
> > 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
> > necessary in a few corner cases. 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. That involves the
> > reprogramming of address decoders (HDM Decoders); in the near future
> > it may also include the device offering dynamic capacity extents. 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.
> >
> > So on other architectures such as ARM64 we have no instruction similar to
> > WBINVD but we may have device interfaces in the system that provide a way
> > to ensure a PA range undergoes the write back and invalidate action. This
> > RFC is to find a way to support those cache maintenance device interfaces.
> > The ones I know about are much more flexible than WBINVD, allowing
> > invalidation of particular PA ranges, or a much richer set of flush types
> > (not supported yet as not needed for upstream use cases).
> >
> > To illustrate how a solution might work, I've taken both a HiSilicon
> > design (slight quirk as registers overlap with existing PMU driver)
> > and more controversially a firmware interface proposal from ARM
> > (wrapped up in made up ACPI) that was dropped from the released spec
> > but for which the alpha spec is still available.
> >
> > Why drivers/cache?
> > - Mainly because it exists and smells like a reasonable place.
> > - Conor, you are maintainer for this currently do you mind us putting this
> > stuff in there?
>
> drivers/cache was just something to put the cache controller drivers we
> have on RISC-V that implement the various arch_dma*() callbacks in
> non-standard ways that made more sense than drivers/soc/<soc vendor>
> since the controllers are IP provided by CPU vendors. There's only
> two drivers here now, but I am aware of another two non-standard CMO
> mechanisms if the silicon with them so there'll likely be more in the
> future :) I'm only really maintainer of it to avoid it being another
> thing for Palmer to look after :)

I suspected as much :)

>
> I've only skimmed this for now, but I think it is reasonable to put them
> here. Maybe my skim is showing, but it would not surprise me to see a
> driver providing both non-standard arch_dma*() callbacks as well as
> dealing with CXL mappings via this new class on RISC-V in the future..

Absolutely. The use of an ARM callback was just a place holder for now
(Greg pointed that one out as well as I forgot to mention it in the patch
description!)

I think this will turn out to be at least some subset of implementations
for other architectures unless they decide to go the route of an instruction
(like x86).

> Either way, I think it'd probably be a good idea to add ?you? as a
> co-maintainer if the directory is going to be used for your proposed
> interface/drivers, for what I hope is an obvious reason!

Sure. That would make sense.

Jonathan
>