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From: Nathan Lynch <nathanl@linux.ibm.com>
To: Kajol Jain <kjain@linux.ibm.com>
Subject: Re: [PATCH v8 2/5] powerpc/hv-24x7: Add rtas call in hv-24x7 driver
 to get processor details
In-Reply-To: <20200506110737.14904-3-kjain@linux.ibm.com>
References: <20200506110737.14904-1-kjain@linux.ibm.com>
 <20200506110737.14904-3-kjain@linux.ibm.com>
Date: Mon, 11 May 2020 16:07:39 -0500
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Sender: "Linuxppc-dev"
 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

Hi,

Kajol Jain <kjain@linux.ibm.com> writes:
> diff --git a/arch/powerpc/perf/hv-24x7.c b/arch/powerpc/perf/hv-24x7.c
> index 48e8f4b17b91..8cf242aad98f 100644
> --- a/arch/powerpc/perf/hv-24x7.c
> +++ b/arch/powerpc/perf/hv-24x7.c
> @@ -20,6 +20,7 @@
>  #include <asm/io.h>
>  #include <linux/byteorder/generic.h>
>  
> +#include <asm/rtas.h>
>  #include "hv-24x7.h"
>  #include "hv-24x7-catalog.h"
>  #include "hv-common.h"
> @@ -57,6 +58,75 @@ static bool is_physical_domain(unsigned domain)
>  	}
>  }
>  
> +/*
> + * The Processor Module Information system parameter allows transferring
> + * of certain processor module information from the platform to the OS.
> + * Refer PAPR+ document to get parameter token value as '43'.
> + */
> +
> +#define PROCESSOR_MODULE_INFO   43
> +#define PROCESSOR_MAX_LENGTH	(8 * 1024)
> +
> +DEFINE_SPINLOCK(rtas_local_data_buf_lock);
> +EXPORT_SYMBOL(rtas_local_data_buf_lock);

This should be static and not exported, correct?

> +
> +static u32 phys_sockets;	/* Physical sockets */
> +static u32 phys_chipspersocket;	/* Physical chips per socket*/
> +static u32 phys_coresperchip; /* Physical cores per chip */
> +
> +/*
> + * Function read_sys_info_pseries() make a rtas_call which require
> + * data buffer of size 8K. As standard 'rtas_data_buf' is of size
> + * 4K, we are adding new local buffer 'rtas_local_data_buf'.

Sorry if this has been covered before but I don't understand why it
would require a larger buffer; by my reading this call will return *ten
bytes* of output. Also, current versions of PAPR+ limit the output
length to 4002 bytes. I feel like I'm missing something.


> + */
> +static __be16 rtas_local_data_buf[PROCESSOR_MAX_LENGTH] __cacheline_aligned;
> +
> +/*
> + * read_sys_info_pseries()
> + * Retrieve the number of sockets and chips per socket and cores per
> + * chip details through the get-system-parameter rtas call.
> + */
> +void read_sys_info_pseries(void)
> +{
> +	int call_status, len, ntypes;
> +
> +	/*
> +	 * Making system parameter: chips and sockets and cores per chip
> +	 * default to 1.
> +	 */
> +	phys_sockets = 1;
> +	phys_chipspersocket = 1;
> +	phys_coresperchip = 1;
> +	memset(rtas_local_data_buf, 0, PROCESSOR_MAX_LENGTH * sizeof(__be16));

Modifying global state outside of any critical section...? How do
you prevent readers from seeing inconsistent results?


> +	spin_lock(&rtas_local_data_buf_lock);
> +
> +	call_status = rtas_call(rtas_token("ibm,get-system-parameter"), 3, 1,
> +				NULL,
> +				PROCESSOR_MODULE_INFO,
> +				__pa(rtas_local_data_buf),
> +				PROCESSOR_MAX_LENGTH);
> +
> +	spin_unlock(&rtas_local_data_buf_lock);

Using this lock this way fails to provide any protection to the data
buffer or the phys_* variables.


> +
> +	if (call_status != 0) {
> +		pr_info("Error calling get-system-parameter (0x%x)\n",
> +			call_status);

To be robust, this should handle busy (-2) and extended delay (990x)
statuses. And if it's going to log errors it should use pr_err() and use
decimal, not hex, to report the RTAS call status, since that's how
they're specified in PAPR+.