linux-kernel - Re: [PATCH v4 9/9] cxl/mem: Add payload dumping for debug

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Date:   Tue, 16 Feb 2021 15:48:57 +0000
From:   Jonathan Cameron <Jonathan.Cameron@...wei.com>
To:     Ben Widawsky <ben.widawsky@...el.com>
CC:     <linux-cxl@...r.kernel.org>, <linux-acpi@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <linux-nvdimm@...ts.01.org>,
        <linux-pci@...r.kernel.org>, Bjorn Helgaas <helgaas@...nel.org>,
        "Chris Browy" <cbrowy@...ry-design.com>,
        Christoph Hellwig <hch@...radead.org>,
        "Dan Williams" <dan.j.williams@...el.com>,
        David Hildenbrand <david@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Ira Weiny <ira.weiny@...el.com>,
        "Jon Masters" <jcm@...masters.org>,
        Rafael Wysocki <rafael.j.wysocki@...el.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Vishal Verma <vishal.l.verma@...el.com>,
        "John Groves (jgroves)" <jgroves@...ron.com>,
        "Kelley, Sean V" <sean.v.kelley@...el.com>
Subject: Re: [PATCH v4 9/9] cxl/mem: Add payload dumping for debug

On Mon, 15 Feb 2021 17:45:38 -0800
Ben Widawsky <ben.widawsky@...el.com> wrote:

> It's often useful in debug scenarios to see what the hardware has dumped
> out. As it stands today, any device error will result in the payload not
> being copied out, so there is no way to triage commands which weren't
> expected to fail (and sometimes the payload may have that information).
> 
> The functionality is protected by normal kernel security mechanisms as
> well as a CONFIG option in the CXL driver.
> 
> This was extracted from the original version of the CXL enabling patch
> series.
> 
> Signed-off-by: Ben Widawsky <ben.widawsky@...el.com>

My gut feeling here is use a tracepoint rather than spamming the kernel
log.  Alternatively just don't bother merging this patch - it's on the list
now anyway so trivial for anyone doing such debug to pick it up.

Jonathan



> ---
>  drivers/cxl/Kconfig | 13 +++++++++++++
>  drivers/cxl/mem.c   |  8 ++++++++
>  2 files changed, 21 insertions(+)
> 
> diff --git a/drivers/cxl/Kconfig b/drivers/cxl/Kconfig
> index 97dc4d751651..3eec9276e586 100644
> --- a/drivers/cxl/Kconfig
> +++ b/drivers/cxl/Kconfig
> @@ -50,4 +50,17 @@ config CXL_MEM_RAW_COMMANDS
>  	  potential impact to memory currently in use by the kernel.
>  
>  	  If developing CXL hardware or the driver say Y, otherwise say N.
> +
> +config CXL_MEM_INSECURE_DEBUG
> +	bool "CXL.mem debugging"
> +	depends on CXL_MEM
> +	help
> +	  Enable debug of all CXL command payloads.
> +
> +	  Some CXL devices and controllers support encryption and other
> +	  security features. The payloads for the commands that enable
> +	  those features may contain sensitive clear-text security
> +	  material. Disable debug of those command payloads by default.
> +	  If you are a kernel developer actively working on CXL
> +	  security enabling say Y, otherwise say N.
>  endif
> diff --git a/drivers/cxl/mem.c b/drivers/cxl/mem.c
> index dc608bb20a31..237b956f0be0 100644
> --- a/drivers/cxl/mem.c
> +++ b/drivers/cxl/mem.c
> @@ -342,6 +342,14 @@ static int __cxl_mem_mbox_send_cmd(struct cxl_mem *cxlm,
>  
>  	/* #5 */
>  	rc = cxl_mem_wait_for_doorbell(cxlm);
> +
> +	if (!cxl_is_security_command(mbox_cmd->opcode) ||
> +	    IS_ENABLED(CONFIG_CXL_MEM_INSECURE_DEBUG)) {
> +		print_hex_dump_debug("Payload ", DUMP_PREFIX_OFFSET, 16, 1,
> +				     mbox_cmd->payload_in, mbox_cmd->size_in,
> +				     true);
> +	}
> +
>  	if (rc == -ETIMEDOUT) {
>  		cxl_mem_mbox_timeout(cxlm, mbox_cmd);
>  		return rc;