Re: perf hw in kexeced kernel broken in tip

[Yinghai Lu]

On 12/08/2010 06:59 AM, Peter Zijlstra wrote:
> On Wed, 2010-12-08 at 15:20 +0100, Peter Zijlstra wrote:
> 
>>> I wonder if you should reverse these checks.  If the bios has the perf
>>> counter enabled, there might be a high chance that it fails the first
>>> check and never gets to the actually bios checks.
>>
>> Ah, good point. 
> 
> Something like so..
> 
> ---
> Subject: perf, x86: Detect broken BIOSes
> From: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> Date: Wed Dec 08 15:56:23 CET 2010
> 
> Some BIOSes use PMU resources, this is a bug.
> 
> Try to detect this, warn about it, and further refuse to touch the
> PMU ourselves.
> 
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> ---
> Index: linux-2.6/arch/x86/kernel/cpu/perf_event.c
> ===================================================================
> --- linux-2.6.orig/arch/x86/kernel/cpu/perf_event.c
> +++ linux-2.6/arch/x86/kernel/cpu/perf_event.c
> @@ -375,15 +375,51 @@ static void release_pmc_hardware(void) {
>  static bool check_hw_exists(void)
>  {
>  	u64 val, val_new = 0;
> -	int ret = 0;
> +	int i, reg, ret = 0;
>  
> +	/*
> +	 * Check to see if the BIOS enabled any of the counters, if so
> +	 * complain and bail.
> +	 */
> +	for (i = 0; i < x86_pmu.num_counters; i++) {
> +		reg = x86_pmu.eventsel + i;
> +		ret = rdmsrl_safe(reg, &val);
> +		if (ret)
> +			goto msr_fail;
> +		if (val & ARCH_PERFMON_EVENTSEL_ENABLE)
> +			goto bios_fail;
> +	}
> +
> +	for (i = 0; i < x86_pmu.num_counters_fixed; i++) {
> +		reg = MSR_ARCH_PERFMON_FIXED_CTR_CTRL;
> +		ret = rdmsrl_safe(reg, &val);
> +		if (ret)
> +			goto msr_fail;
> +		if (val & (0x03 << i*4))
> +			goto bios_fail;
> +	}
> +
> +	/*
> +	 * Now write a value and read it back to see if it matches,
> +	 * this is needed to detect certain hardware emulators (qemu/kvm)
> +	 * that don't trap on the MSR access and always return 0s.
> +	 */
>  	val = 0xabcdUL;
> -	ret |= checking_wrmsrl(x86_pmu.perfctr, val);
> +	ret = checking_wrmsrl(x86_pmu.perfctr, val);
>  	ret |= rdmsrl_safe(x86_pmu.perfctr, &val_new);
>  	if (ret || val != val_new)
> -		return false;
> +		goto msr_fail;
>  
>  	return true;
> +
> +bios_fail:
> +	printk(KERN_CONT "Broken BIOS detected, software events only.\n");
> +	printk(KERN_ERR FW_BUG "invalid MSR: %x=%Lx\n", reg, val);
> +	return false;
> +
> +msr_fail:
> +	printk(KERN_CONT "Broken PMU hardware detected, software events only.\n");
> +	return false;
>  }

can you add sth force_... in command line to take over ownership of perf from BIOS or previous kernel ?

then still can use perf etc after we kexec from RHEL or SLES kernel to later kernel ( from 2.6.37)

Thanks

Yinghai

>  
>  static void reserve_ds_buffers(void);
> @@ -1378,10 +1414,8 @@ int __init init_hw_perf_events(void)
>  	pmu_check_apic();
>  
>  	/* sanity check that the hardware exists or is emulated */
> -	if (!check_hw_exists()) {
> -		pr_cont("Broken PMU hardware detected, software events only.\n");
> +	if (!check_hw_exists())
>  		return 0;
> -	}
>  
>  	pr_cont("%s PMU driver.\n", x86_pmu.name);
>  
> 

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