Re: [PATCH 1/2] KVM: arm64: Turn kvm_arm_support_pmu_v3() into a static key

From: Marc Zyngier <maz@kernel.org>
To: Andre Przywara <andre.przywara@arm.com>
Cc: Suzuki K Poulose <suzuki.poulose@arm.com>,
	James Morse <james.morse@arm.com>,
	linux-arm-kernel@lists.infradead.org, kernel-team@android.com,
	kvmarm@lists.cs.columbia.edu,
	Julien Thierry <julien.thierry.kdev@gmail.com>
Subject: Re: [PATCH 1/2] KVM: arm64: Turn kvm_arm_support_pmu_v3() into a static key
Date: Thu, 28 Jan 2021 16:56:01 +0000	[thread overview]
Message-ID: <9bf9be4149cfd0671fe5178ce553159b@kernel.org> (raw)
In-Reply-To: <20210128151643.6e2c3668@slackpad.fritz.box>

On 2021-01-28 15:16, Andre Przywara wrote:
> On Tue, 26 Jan 2021 15:15:20 +0000
> Marc Zyngier <maz@kernel.org> wrote:
> 
> Hi Marc,
> 
>> We currently find out about the presence of a HW PMU (or the handling
>> of that PMU by perf, which amounts to the same thing) in a fairly
>> roundabout way, by checking the number of counters available to perf.
>> That's good enough for now, but we will soon need to find about about
>> that on paths where perf is out of reach (in the world switch).
>> 
>> Instead, let's turn kvm_arm_support_pmu_v3() into a static key.
> 
> I am sure the pesky build bot has told you about it already, but this
> fails when ARM_PMU is not defined, as perf_num_counters() is not
> defined. It's  bit nasty, since it's a generic function, so we
> can't easily stub it in its original header.

No sign from the bot yet, but that's indeed a problem. Well spotted.

> Shall we find a place somewhere in arch/arm64 and provide a stub
> implementation there, #ifndef CONFIG_ARM_PMU? Sounds ugly, though.
> 
> Or something else entirely?

How about:

diff --git a/arch/arm64/kvm/perf.c b/arch/arm64/kvm/perf.c
index 198fa4266b2d..739164324afe 100644
--- a/arch/arm64/kvm/perf.c
+++ b/arch/arm64/kvm/perf.c
@@ -55,7 +55,7 @@ int kvm_perf_init(void)
  	 * hardware performance counters. This could ensure the presence of
  	 * a physical PMU and CONFIG_PERF_EVENT is selected.
  	 */
-	if (perf_num_counters() > 0)
+	if (IS_ENABLED(CONFIG_ARM_PMU) && perf_num_counters() > 0)
  		static_branch_enable(&kvm_arm_pmu_available);

  	return perf_register_guest_info_callbacks(&kvm_guest_cbs);

It certainly compiles here.

         M.
-- 
Jazz is not dead. It just smells funny...

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