Re: [PATCH v25 10/11] cxl: Avoid dax creation for accelerators

[Dan Williams <dan.j.williams@intel.com>]

alejandro.lucero-palau@ wrote:
> From: Alejandro Lucero <alucerop@amd.com>
> 
> By definition a type2 cxl device will use the host managed memory for
> specific functionality, therefore it should not be available to other
> uses like DAX.
> 
> Signed-off-by: Alejandro Lucero <alucerop@amd.com>
> Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> Reviewed-by: Davidlohr Bueso <daves@stgolabs.net>
> Reviewed-by: Dave Jiang <dave.jiang@intel.com>
> Reviewed-by: Ben Cheatham <benjamin.cheatham@amd.com>
> ---
>  drivers/cxl/core/region.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
> index b3a6d08be461..6a89a1817199 100644
> --- a/drivers/cxl/core/region.c
> +++ b/drivers/cxl/core/region.c
> @@ -4264,6 +4264,13 @@ static int cxl_region_probe(struct device *dev)
>  	if (rc)
>  		return rc;
>  
> +	/*
> +	 * HDM-D[B] (device-memory) regions have accelerator specific usage.
> +	 * Skip device-dax registration.
> +	 */
> +	if (cxlr->type == CXL_DECODER_DEVMEM)
> +		return 0;
> +

It is possible for a general memory expander to also be
CXL_DECODER_DEVMEM. Imagine a memory expander that could back
invalidate on decoder change. See my reply to patch5 for the changes to
cxl_region_probe() when an accelerator driver wants to own mapping a
resulting region.

Also note that cxl_region_has_memdev_attach() arranges for memory
notifiers and poison to be setup. This is for accelerators that may use
devm_memremap_pages() and accelerators that may implement a full CXL
mailbox for whatever reason. A CXL mailbox remains the only generic
mechanism for determining the DPA layout of a device.