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Date:   Thu, 11 Feb 2021 09:55:48 +0000
From:   Jonathan Cameron <Jonathan.Cameron@Huawei.com>
To:     Ben Widawsky <ben.widawsky@intel.com>
CC:     <linux-cxl@vger.kernel.org>, <linux-acpi@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>, <linux-nvdimm@lists.01.org>,
        <linux-pci@vger.kernel.org>, Bjorn Helgaas <helgaas@kernel.org>,
        "Chris Browy" <cbrowy@avery-design.com>,
        Christoph Hellwig <hch@infradead.org>,
        "Dan Williams" <dan.j.williams@intel.com>,
        David Hildenbrand <david@redhat.com>,
        David Rientjes <rientjes@google.com>,
        Ira Weiny <ira.weiny@intel.com>,
        "Jon Masters" <jcm@jonmasters.org>,
        Rafael Wysocki <rafael.j.wysocki@intel.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Vishal Verma <vishal.l.verma@intel.com>,
        "John Groves (jgroves)" <jgroves@micron.com>,
        "Kelley, Sean V" <sean.v.kelley@intel.com>
Subject: Re: [PATCH v2 2/8] cxl/mem: Find device capabilities
Message-ID: <20210211095548.00000da7@Huawei.com>
In-Reply-To: <20210210181605.ecbl3m5ep4rszpqs@intel.com>
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        <20210210000259.635748-3-ben.widawsky@intel.com>
        <20210210133252.000047af@Huawei.com>
        <20210210150759.00005684@Huawei.com>
        <20210210165557.7fuqbyr7e7zjoxaa@intel.com>
        <20210210181605.ecbl3m5ep4rszpqs@intel.com>
Organization: Huawei Technologies Research and Development (UK) Ltd.
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On Wed, 10 Feb 2021 10:16:05 -0800
Ben Widawsky <ben.widawsky@intel.com> wrote:

> On 21-02-10 08:55:57, Ben Widawsky wrote:
> > On 21-02-10 15:07:59, Jonathan Cameron wrote:  
> > > On Wed, 10 Feb 2021 13:32:52 +0000
> > > Jonathan Cameron <Jonathan.Cameron@Huawei.com> wrote:
> > >   
> > > > On Tue, 9 Feb 2021 16:02:53 -0800
> > > > Ben Widawsky <ben.widawsky@intel.com> wrote:
> > > >   
> > > > > Provide enough functionality to utilize the mailbox of a memory device.
> > > > > The mailbox is used to interact with the firmware running on the memory
> > > > > device. The flow is proven with one implemented command, "identify".
> > > > > Because the class code has already told the driver this is a memory
> > > > > device and the identify command is mandatory.
> > > > > 
> > > > > CXL devices contain an array of capabilities that describe the
> > > > > interactions software can have with the device or firmware running on
> > > > > the device. A CXL compliant device must implement the device status and
> > > > > the mailbox capability. Additionally, a CXL compliant memory device must
> > > > > implement the memory device capability. Each of the capabilities can
> > > > > [will] provide an offset within the MMIO region for interacting with the
> > > > > CXL device.
> > > > > 
> > > > > The capabilities tell the driver how to find and map the register space
> > > > > for CXL Memory Devices. The registers are required to utilize the CXL
> > > > > spec defined mailbox interface. The spec outlines two mailboxes, primary
> > > > > and secondary. The secondary mailbox is earmarked for system firmware,
> > > > > and not handled in this driver.
> > > > > 
> > > > > Primary mailboxes are capable of generating an interrupt when submitting
> > > > > a background command. That implementation is saved for a later time.
> > > > > 
> > > > > Link: https://www.computeexpresslink.org/download-the-specification
> > > > > Signed-off-by: Ben Widawsky <ben.widawsky@intel.com>
> > > > > Reviewed-by: Dan Williams <dan.j.williams@intel.com>    
> > > > 
> > > > Hi Ben,
> > > > 
> > > >   
> > > > > +/**
> > > > > + * cxl_mem_mbox_send_cmd() - Send a mailbox command to a memory device.
> > > > > + * @cxlm: The CXL memory device to communicate with.
> > > > > + * @mbox_cmd: Command to send to the memory device.
> > > > > + *
> > > > > + * Context: Any context. Expects mbox_lock to be held.
> > > > > + * Return: -ETIMEDOUT if timeout occurred waiting for completion. 0 on success.
> > > > > + *         Caller should check the return code in @mbox_cmd to make sure it
> > > > > + *         succeeded.    
> > > > 
> > > > cxl_xfer_log() doesn't check mbox_cmd->return_code and for my test it currently
> > > > enters an infinite loop as a result.  
> > 
> > I meant to fix that.
> >   
> > > > 
> > > > I haven't checked other paths, but to my mind it is not a good idea to require
> > > > two levels of error checking - the example here proves how easy it is to forget
> > > > one.  
> > 
> > Demonstrably, you're correct. I think it would be good to have a kernel only
> > mbox command that does the error checking though. Let me type something up and
> > see how it looks.  
> 
> Hi Jonathan. What do you think of this? The bit I'm on the fence about is if I
> should validate output size too. I like the simplicity as it is, but it requires
> every caller to possibly check output size, which is kind of the same problem
> you're originally pointing out.

The simplicity is good and this is pretty much what I expected you would end up with
(always reassuring)

For the output, perhaps just add another parameter to the wrapper for minimum
output length expected?

Now you mention the length question.  It does rather feel like there should also
be some protection on memcpy_fromio() copying too much data if the hardware
happens to return an unexpectedly long length.  Should never happen, but
the hardening is worth adding anyway given it's easy to do.

Jonathan


> 
> diff --git a/drivers/cxl/mem.c b/drivers/cxl/mem.c
> index 55c5f5a6023f..ad7b2077ab28 100644
> --- a/drivers/cxl/mem.c
> +++ b/drivers/cxl/mem.c
> @@ -284,7 +284,7 @@ static void cxl_mem_mbox_timeout(struct cxl_mem *cxlm,
>  }
>  
>  /**
> - * cxl_mem_mbox_send_cmd() - Send a mailbox command to a memory device.
> + * __cxl_mem_mbox_send_cmd() - Execute a mailbox command
>   * @cxlm: The CXL memory device to communicate with.
>   * @mbox_cmd: Command to send to the memory device.
>   *
> @@ -296,7 +296,8 @@ static void cxl_mem_mbox_timeout(struct cxl_mem *cxlm,
>   * This is a generic form of the CXL mailbox send command, thus the only I/O
>   * operations used are cxl_read_mbox_reg(). Memory devices, and perhaps other
>   * types of CXL devices may have further information available upon error
> - * conditions.
> + * conditions. Driver facilities wishing to send mailbox commands should use the
> + * wrapper command.
>   *
>   * The CXL spec allows for up to two mailboxes. The intention is for the primary
>   * mailbox to be OS controlled and the secondary mailbox to be used by system
> @@ -304,8 +305,8 @@ static void cxl_mem_mbox_timeout(struct cxl_mem *cxlm,
>   * not need to coordinate with each other. The driver only uses the primary
>   * mailbox.
>   */
> -static int cxl_mem_mbox_send_cmd(struct cxl_mem *cxlm,
> -				 struct mbox_cmd *mbox_cmd)
> +static int __cxl_mem_mbox_send_cmd(struct cxl_mem *cxlm,
> +				   struct mbox_cmd *mbox_cmd)
>  {
>  	void __iomem *payload = cxlm->mbox_regs + CXLDEV_MBOX_PAYLOAD_OFFSET;
>  	u64 cmd_reg, status_reg;
> @@ -469,6 +470,54 @@ static void cxl_mem_mbox_put(struct cxl_mem *cxlm)
>  	mutex_unlock(&cxlm->mbox_mutex);
>  }
>  
> +/**
> + * cxl_mem_mbox_send_cmd() - Send a mailbox command to a memory device.
> + * @cxlm: The CXL memory device to communicate with.
> + * @opcode: Opcode for the mailbox command.
> + * @in: The input payload for the mailbox command.
> + * @in_size: The length of the input payload
> + * @out: Caller allocated buffer for the output.
> + *
> + * Context: Any context. Will acquire and release mbox_mutex.
> + * Return:
> + *  * %>=0	- Number of bytes returned in @out.
> + *  * %-EBUSY	- Couldn't acquire exclusive mailbox access.
> + *  * %-EFAULT	- Hardware error occurred.
> + *  * %-ENXIO	- Command completed, but device reported an error.
> + *
> + * Mailbox commands may execute successfully yet the device itself reported an
> + * error. While this distinction can be useful for commands from userspace, the
> + * kernel will often only care when both are successful.
> + *
> + * See __cxl_mem_mbox_send_cmd()
> + */
> +static int cxl_mem_mbox_send_cmd(struct cxl_mem *cxlm, u16 opcode, u8 *in,
> +				 size_t in_size, u8 *out)
> +{
> +	struct mbox_cmd mbox_cmd = {
> +		.opcode = opcode,
> +		.payload_in = in,
> +		.size_in = in_size,
> +		.payload_out = out,
> +	};
> +	int rc;
> +
> +	rc = cxl_mem_mbox_get(cxlm);
> +	if (rc)
> +		return rc;
> +
> +	rc = __cxl_mem_mbox_send_cmd(cxlm, &mbox_cmd);
> +	cxl_mem_mbox_put(cxlm);
> +	if (rc)
> +		return rc;
> +
> +	/* TODO: Map return code to proper kernel style errno */
> +	if (mbox_cmd.return_code != CXL_MBOX_SUCCESS)
> +		return -ENXIO;
> +
> +	return mbox_cmd.size_out;
> +}
> +
>  /**
>   * handle_mailbox_cmd_from_user() - Dispatch a mailbox command.
>   * @cxlmd: The CXL memory device to communicate with.
> @@ -1380,33 +1429,18 @@ static int cxl_mem_identify(struct cxl_mem *cxlm)
>  		u8 poison_caps;
>  		u8 qos_telemetry_caps;
>  	} __packed id;
> -	struct mbox_cmd mbox_cmd = {
> -		.opcode = CXL_MBOX_OP_IDENTIFY,
> -		.payload_out = &id,
> -		.size_in = 0,
> -	};
>  	int rc;
>  
> -	/* Retrieve initial device memory map */
> -	rc = cxl_mem_mbox_get(cxlm);
> -	if (rc)
> -		return rc;
> -
> -	rc = cxl_mem_mbox_send_cmd(cxlm, &mbox_cmd);
> -	cxl_mem_mbox_put(cxlm);
> -	if (rc)
> +	rc = cxl_mem_mbox_send_cmd(cxlm, CXL_MBOX_OP_IDENTIFY, NULL, 0,
> +				   (u8 *)&id);
> +	if (rc < 0)
>  		return rc;
>  
> -	/* TODO: Handle retry or reset responses from firmware. */
> -	if (mbox_cmd.return_code != CXL_MBOX_SUCCESS) {
> -		dev_err(&cxlm->pdev->dev, "Mailbox command failed (%d)\n",
> -			mbox_cmd.return_code);
> +	if (rc < sizeof(id)) {
> +		dev_err(&cxlm->pdev->dev, "Short identify data\n",
>  		return -ENXIO;
>  	}
>  
> -	if (mbox_cmd.size_out != sizeof(id))
> -		return -ENXIO;
> -
>  	/*
>  	 * TODO: enumerate DPA map, as 'ram' and 'pmem' do not alias.
>  	 * For now, only the capacity is exported in sysfs
> 
> 
> [snip]
>