netdev - Re: [PATCH v16 19/22] cxl: Add region flag for precluding a device memory to be used for dax

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Message-ID: <682e3c7129fb0_1626e10062@dwillia2-xfh.jf.intel.com.notmuch>
Date: Wed, 21 May 2025 13:49:53 -0700
From: Dan Williams <dan.j.williams@...el.com>
To: <alejandro.lucero-palau@....com>, <linux-cxl@...r.kernel.org>,
	<netdev@...r.kernel.org>, <dan.j.williams@...el.com>, <edward.cree@....com>,
	<davem@...emloft.net>, <kuba@...nel.org>, <pabeni@...hat.com>,
	<edumazet@...gle.com>, <dave.jiang@...el.com>
CC: Alejandro Lucero <alucerop@....com>, Zhi Wang <zhiw@...dia.com>, "Jonathan
 Cameron" <Jonathan.Cameron@...wei.com>, Ben Cheatham
	<benjamin.cheatham@....com>
Subject: Re: [PATCH v16 19/22] cxl: Add region flag for precluding a device
 memory to be used for dax

alejandro.lucero-palau@ wrote:
> From: Alejandro Lucero <alucerop@....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. However, a dax interface could be just good enough in some cases.
> 
> Add a flag to a cxl region for specifically state to not create a dax
> device. Allow a Type2 driver to set that flag at region creation time.
> 
> Signed-off-by: Alejandro Lucero <alucerop@....com>
> Reviewed-by: Zhi Wang <zhiw@...dia.com>
> Reviewed-by: Jonathan Cameron <Jonathan.Cameron@...wei.com>
> Reviewed-by: Ben Cheatham <benjamin.cheatham@....com>
> Reviewed-by: Dave Jiang <dave.jiang@...el.com>
> ---

What was wrong with the original proposal?

+
+		/*
+		 * HDM-D[B] (device-memory) regions have accelerator
+		 * specific usage, skip device-dax registration.
+		 */
+		if (cxlr->type == CXL_DECODER_DEVMEM)
+			return 0;

I really do not want the ABI presentation policy layer leaking that deep into
the region creation flow. Another way to determine this is if devices
hosting the region are not driver by the generic CXL device memory class
driver 'cxl_pci'.