linux-kernel - Re: [PATCH v3] powerpc/pseries: read the lpar name from the firmware

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Message-ID: <bbaa0d78-a09f-3ce3-25a9-67434039b741@linux.ibm.com>
Date:   Tue, 7 Dec 2021 17:07:29 +0100
From:   Laurent Dufour <ldufour@...ux.ibm.com>
To:     Nathan Lynch <nathanl@...ux.ibm.com>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v3] powerpc/pseries: read the lpar name from the firmware

On 07/12/2021, 15:32:39, Nathan Lynch wrote:
> Hi Laurent,
> 
> Laurent Dufour <ldufour@...ux.ibm.com> writes:
>> +/*
>> + * PAPR defines, in section "7.3.16 System Parameters Option", the token 55 to
>> + * read the LPAR name.
>> + */
>> +#define SPLPAR_LPAR_NAME_TOKEN	55
>> +static void read_lpar_name(struct seq_file *m)
>> +{
>> +	int rc, len, token;
>> +	union {
>> +		char raw_buffer[RTAS_DATA_BUF_SIZE];
>> +		struct {
>> +			__be16 len;
> 
> This:
> 
>> +			char name[RTAS_DATA_BUF_SIZE-2];
>                                        ^^^^^^
> 
> should be 4000, not (4K - 2), according to PAPR (it's weird and I don't
> know the reason).

That's true, PAPR defines the largest output buffer for
ibm,get-system-parameter to 4002, so we could limit this to 4002, not sure
whether this would make a big difference here. Anyway I will limit that
buffer size this way.
> 
> 
>> +		};
>> +	} *local_buffer;
>> +
>> +	token = rtas_token("ibm,get-system-parameter");
>> +	if (token == RTAS_UNKNOWN_SERVICE)
>> +		return;
>> +
>> +	local_buffer = kmalloc(sizeof(*local_buffer), GFP_KERNEL);
>> +	if (!local_buffer)
>> +		return;
>> +
>> +	do {
>> +		spin_lock(&rtas_data_buf_lock);
>> +		memset(rtas_data_buf, 0, RTAS_DATA_BUF_SIZE);
>> +		rc = rtas_call(token, 3, 1, NULL, SPLPAR_LPAR_NAME_TOKEN,
>> +			       __pa(rtas_data_buf), RTAS_DATA_BUF_SIZE);
>> +		if (!rc)
>> +			memcpy(local_buffer->raw_buffer, rtas_data_buf,
>> +			       RTAS_DATA_BUF_SIZE);
>> +		spin_unlock(&rtas_data_buf_lock);
>> +	} while (rtas_busy_delay(rc));
>> +
>> +	if (rc != 0) {
>> +		pr_err_once(
>> +			"%s %s Error calling get-system-parameter (0x%x)\n",
>> +			__FILE__, __func__, rc);
> 
> The __FILE__ and __func__ in the message seem unnecessary, and rc should
> be reported in decimal so the error meaning is apparent.

Fair enough.

> Is there a reasonable fallback for VMs where this parameter doesn't
> exist? PowerVM partitions should always have it, but what do we want the
> behavior to be on other hypervisors?

In that case, there is no value displayed in the /proc/powerpc/lparcfg and
the lparstat -i command will fall back to the device tree value. I can't
see any valid reason to report the value defined in the device tree here.

> 
> 
>> +	} else {
>> +		/* Force end of string */
>> +		len = be16_to_cpu(local_buffer->len);
>> +		if (len >= (RTAS_DATA_BUF_SIZE-2))
>> +			len = RTAS_DATA_BUF_SIZE-2;
> 
> Could use min() or clamp(), and it would be better to build the
> expression using the value of sizeof(local_buffer->name).

Fair enough.

> 
>> +		local_buffer->name[len] = '\0';
> 
> If 'len' can be (RTAS_DATA_BUF_SIZE - 2), then this writes past the end
> of the buffer, no?

Oh yes, the previous test should be
		if (len >= (RTAS_DATA_BUF_SIZE-2))
			len = RTAS_DATA_BUF_SIZE-3;

Thanks,
Laurent.