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Date:	Mon, 14 Jul 2014 13:08:08 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	kan.liang@...el.com
Cc:	andi@...stfloor.org, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Subject: Re: [PATCH V5 1/2] perf ignore LBR and extra_regs

On Thu, Jul 10, 2014 at 03:59:43AM -0700, kan.liang@...el.com wrote:
> +/*
> + * Under certain circumstances, access certain MSR may cause #GP.
> + * The function tests if the input MSR can be safely accessed.
> + */
> +static inline bool check_msr(unsigned long msr)
> +{
> +	u64 val_old, val_new, val_tmp;
> +
> +	/*
> +	 * Read the current value, change it 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.
> +	 */
> +	if (rdmsrl_safe(msr, &val_old))
> +		goto msr_fail;
> +	/*
> +	 * Only chagne it slightly,
> +	 * since the higher bits of some MSRs cannot be updated by wrmsrl.
> +	 * E.g. MSR_LBR_TOS
> +	 */
> +	val_tmp = val_old ^ 0x3UL;
> +	if (wrmsrl_safe(msr, val_tmp) ||
> +		rdmsrl_safe(msr, &val_new))
> +		goto msr_fail;
> +
> +	if (val_new != val_tmp)
> +		goto msr_fail;
> +
> +	/* Here it's sure that the MSR can be safely accessed.
> +	 * Restore the old value and return.
> +	 */
> +	wrmsrl(msr, val_old);
> +
> +	return true;
> +
> +msr_fail:
> +	return false;
> +}

I don't think we need the msr_fail thing, there's no state to clean up,
you can return false at all places you now have goto.

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