Date:   Fri, 12 Feb 2021 07:54:35 -0800
From:   Ben Widawsky <ben.widawsky@...el.com>
To:     Jonathan Cameron <Jonathan.Cameron@...wei.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 v2 2/8] cxl/mem: Find device capabilities

On 21-02-12 13:27:06, Jonathan Cameron wrote:
> On Thu, 11 Feb 2021 07:55:29 -0800
> Ben Widawsky <ben.widawsky@...el.com> wrote:
> 
> > On 21-02-11 09:55:48, Jonathan Cameron wrote:
> > > On Wed, 10 Feb 2021 10:16:05 -0800
> > > Ben Widawsky <ben.widawsky@...el.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@...wei.com> wrote:
> > > > > >     
> > > > > > > On Tue, 9 Feb 2021 16:02:53 -0800
> > > > > > > Ben Widawsky <ben.widawsky@...el.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@...el.com>
> > > > > > > > Reviewed-by: Dan Williams <dan.j.williams@...el.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  
> > 
> > Some background because I forget what I've said previously... It's unfortunate
> > that the spec maxes at 1M mailbox size but has enough bits in the length field
> > to support 2M-1. I've made some requests to have this fixed, so maybe 3.0 won't
> > be awkward like this.
> 
> Agreed spec should be tighter here, but I'd argue over 1M indicates buggy hardware.
> 
> > 
> > I think it makes sense to do as you suggested. One question though, do you have
> > an opinion on we return to the caller as the output payload size, do we cap it
> > at 1M also, or are we honest?
> > 
> > -       if (out_len && mbox_cmd->payload_out)
> > -               memcpy_fromio(mbox_cmd->payload_out, payload, out_len);
> > +       if (out_len && mbox_cmd->payload_out) {
> > +               size_t n = min_t(size_t, cxlm->payload_size, out_len);
> > +               memcpy_fromio(mbox_cmd->payload_out, payload, n);
> > +       }
> 
> Ah, I read emails in wrong order.  What you have is what I expected and got
> confused about in your other email.
> 
> > 
> > So...
> > mbox_cmd->size_out = out_len;
> > mbox_cmd->size_out = n;
> 
> Good question.  My gut says the second one.
> Maybe it's worth a warning print to let us know something
> unexpected happened.
> 

I also prefer 'n', It's unfortunate though if userspace hits this condition, it
would have to scrape kernel logs to find out. Perhaps though userspace wouldn't
ever really care.

> > 
> > 
> > > 
> > >   
> > > > 
> > > > 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]
> > > >   
> > >   
> 

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