linux-kernel - Re: [PATCH] perf/x86: descriptive failure messages for PMU init

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Message-ID: <20190415120444.GN11158@hirez.programming.kicks-ass.net>
Date:   Mon, 15 Apr 2019 14:04:44 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Bandan Das <bsd@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org
Subject: Re: [PATCH] perf/x86: descriptive failure messages for PMU init

On Fri, Apr 12, 2019 at 03:09:17PM -0400, Bandan Das wrote:
> 
> There's a default warning message that gets printed, however,
> there are various failure conditions:
>  - a msr read can fail
>  - a msr write can fail
>  - a msr has an unexpected value
>  - all msrs have unexpected values (disable PMU)
> 
> Also, commit commit 005bd0077a79 ("perf/x86: Modify error message in
> virtualized environment") completely removed printing the msr in
> question but these messages could be helpful for debugging vPMUs as
> well. Add them back and change them to pr_debugs, this keeps the
> behavior the same for baremetal.
> 
> Lastly, use %llx to silence checkpatch

Yuck... if you're debugging a hypervisor, you can bloody well run your
own kernel with additional print slattered around.

The whole make an exception for virt bullshit was already pushing it,
this is just insane.

> @@ -266,12 +282,30 @@ static bool check_hw_exists(void)
>  	return true;
>  
>  msr_fail:
> -	if (boot_cpu_has(X86_FEATURE_HYPERVISOR)) {
> +	if (virt)
>  		pr_cont("PMU not available due to virtualization, using software events only.\n");
> -	} else {
> -		pr_cont("Broken PMU hardware detected, using software events only.\n");
> -		pr_err("Failed to access perfctr msr (MSR %x is %Lx)\n",
> -		       reg, val_new);
> +	switch (status) {
> +	case READ_FAIL:
> +		if (virt)
> +			pr_debug("Failed to read perfctr msr (MSR %x)\n", reg);
> +		else
> +			pr_err("Failed to read perfctr msr (MSR %x)\n", reg);
> +		break;
> +	case WRITE_FAIL:
> +		if (virt)
> +			pr_debug("Failed to write perfctr msr (MSR %x, wrote: %llx, read: %llx)\n",
> +				 reg, val, val_new);
> +		else
> +			pr_err("Failed to write perfctr msr (MSR %x, wrote: %llx, read: %llx)\n",
> +				 reg, val, val_new);
> +		break;
> +	case PMU_FAIL:
> +		/* fall through for default message */
> +	default:
> +		if (virt)
> +			pr_debug("Broken PMU hardware detected, using software events only.\n");
> +		else
> +			pr_cont("Broken PMU hardware detected, using software events only.\n");
>  	}