linux-kernel - Re: [PATCH] KVM: x86: disable halt polling when powersave governor is used

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Message-ID: <Y1gXseyl0f3IUnDh@google.com>
Date:   Tue, 25 Oct 2022 17:06:57 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     "Mi, Dapeng1" <dapeng1.mi@...el.com>
Cc:     "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "zhenyuw@...ux.intel.com" <zhenyuw@...ux.intel.com>
Subject: Re: [PATCH] KVM: x86: disable halt polling when powersave governor
 is used

On Sat, Oct 08, 2022, Mi, Dapeng1 wrote:
> > > +				!strncmp(policy->governor->name,
> > "powersave",
> > 
> > KVM should not be comparing magic strings.  If the cpufreq subsystem can't get
> > policy->policy right, then that needs to be fixed.
> 
> Yeah, using magic strings looks a little bit strange, but this is what is
> cpufreq doing.  Currently cpufreq mechanism supports two kinds of drivers,
> one is the driver which has the built-in governor, like intel_pstate driver.
> For this kind of driver, the cpufreq governor is saved in the policy->policy
> field. The other is the traditional driver which is independent with cpufreq
> governor and the cpufreq governor type is saved in the governor->name field.
> For the second kind of cpufreq driver, the policy->policy field is
> meaningless and we have to read the governor name. 

That doesn't mean it's ok to bleed those internal details into KVM.  I would much
rather cpufreq provide a helper to get the effective policy, e.g.

  unsigned int cpufreq_cpu_get_policy(unsigned int cpu)
  {
	struct cpufreq_policy *policy = cpufreq_cpu_get(cpu);
	unsigned int pol;

	if (!policy)
		return CPUFREQ_POLICY_UNKNOWN;

	pol = policy->policy
	if (pol == CPUFREQ_POLICY_UNKNOWN && policy->governor)
		pol = cpufreq_parse_policy(policy->governor->name);

	cpufreq_cpu_put(policy);
  }