linux-kernel - Re: [PATCH v2] hwmon: coretemp: avoid RDMSR interruptions to isolated CPUs

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Date:   Fri, 16 Dec 2022 09:56:20 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     Fenghua Yu <fenghua.yu@...el.com>, linux-kernel@...r.kernel.org,
        linux-hwmon@...r.kernel.org,
        Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [PATCH v2] hwmon: coretemp: avoid RDMSR interruptions to isolated
 CPUs

On 12/16/22 06:07, Marcelo Tosatti wrote:
> 
> The coretemp driver uses rdmsr_on_cpu calls to read
> MSR_IA32_PACKAGE_THERM_STATUS/MSR_IA32_THERM_STATUS registers,
> which contain information about current core temperature.
> 
> For certain low latency applications, the RDMSR interruption exceeds
> the applications requirements.
> 
> So disallow reading of crit_alarm and temp files via /sys, returning
> -EINVAL, in case CPU isolation is enabled.
> 
> Temperature information from the housekeeping cores should be
> sufficient to infer die temperature.
> 
> Signed-off-by: Marcelo Tosatti <mtosatti@...hat.com>
> 
> ---
> v2: improve changelog to mention that an error is returned,
>      and sysfs file is not disabled (Guenter Roeck)
> 

You did not address my feedback. I requested a code change.
Returning -EINVAL is unacceptable, and a solution not creating
the sysfs attributes to start with would be preferred.

Guenter

> diff --git a/drivers/hwmon/coretemp.c b/drivers/hwmon/coretemp.c
> index 9bee4d33fbdf..30a35f4130d5 100644
> --- a/drivers/hwmon/coretemp.c
> +++ b/drivers/hwmon/coretemp.c
> @@ -27,6 +27,7 @@
>   #include <asm/msr.h>
>   #include <asm/processor.h>
>   #include <asm/cpu_device_id.h>
> +#include <linux/sched/isolation.h>
>   
>   #define DRVNAME	"coretemp"
>   
> @@ -121,6 +122,10 @@ static ssize_t show_crit_alarm(struct device *dev,
>   	struct platform_data *pdata = dev_get_drvdata(dev);
>   	struct temp_data *tdata = pdata->core_data[attr->index];
>   
> +
> +	if (!housekeeping_cpu(tdata->cpu, HK_TYPE_MISC))
> +		return -EINVAL;
> +
>   	mutex_lock(&tdata->update_lock);
>   	rdmsr_on_cpu(tdata->cpu, tdata->status_reg, &eax, &edx);
>   	mutex_unlock(&tdata->update_lock);
> @@ -158,6 +163,8 @@ static ssize_t show_temp(struct device *dev,
>   
>   	/* Check whether the time interval has elapsed */
>   	if (!tdata->valid || time_after(jiffies, tdata->last_updated + HZ)) {
> +		if (!housekeeping_cpu(tdata->cpu, HK_TYPE_MISC))
> +			return -EINVAL;
>   		rdmsr_on_cpu(tdata->cpu, tdata->status_reg, &eax, &edx);
>   		/*
>   		 * Ignore the valid bit. In all observed cases the register
> 
> 
> 
>