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

Conor Dooley
2025-03-21T22:34:19Z

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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'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..
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!

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