linux-kernel - Re: [PATCH: kvm 2/6] Kill the confusing tsc_ref_khz and ref

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Date:	Thu, 24 Sep 2009 12:10:49 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Zachary Amsden <zamsden@...hat.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Avi Kivity <avi@...hat.com>
Subject: Re: [PATCH: kvm 2/6] Kill the confusing tsc_ref_khz and ref_freq
	variables.

On Wed, Sep 23, 2009 at 05:29:01PM -1000, Zachary Amsden wrote:
> They are globals, not clearly protected by any ordering or locking, and
> vulnerable to various startup races.
> 
> Instead, for variable TSC machines, register the cpufreq notifier and get
> the TSC frequency directly from the cpufreq machinery.  Not only is it
> always right, it is also perfectly accurate, as no error prone measurement
> is required.  On such machines, also detect the frequency when bringing
> a new CPU online; it isn't clear what frequency it will start with, and
> it may not correspond to the reference.
> 
> Signed-off-by: Zachary Amsden <zamsden@...hat.com>
> ---
>  arch/x86/kvm/x86.c |   38 ++++++++++++++++++++++++++++----------
>  1 files changed, 28 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 15d2ace..35082dd 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -650,6 +650,19 @@ static void kvm_set_time_scale(uint32_t tsc_khz, struct pvclock_vcpu_time_info *
>  
>  static DEFINE_PER_CPU(unsigned long, cpu_tsc_khz);
>  
> +static inline void kvm_get_cpu_khz(int cpu)
> +{
> +	unsigned int khz = cpufreq_get(cpu);

cpufreq_get does down_read, while kvm_arch_hardware_enable is called
either with a spinlock held or from interrupt context?

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