linux-kernel - Re: [PATCH v2 6/7] KVM: x86: Introduce definitions to support static calls for kvm_pmu

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Message-ID: <YbD691K7B9VVbswI@google.com>
Date:   Wed, 8 Dec 2021 18:35:35 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Joerg Roedel <joro@...tes.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 6/7] KVM: x86: Introduce definitions to support static
 calls for kvm_pmu_ops

On Mon, Nov 08, 2021, Like Xu wrote:
> From: Like Xu <likexu@...cent.com>
> 
> Use static calls to improve kvm_pmu_ops performance. Introduce the
> definitions that will be used by a subsequent patch to actualize the
> savings. Add a new kvm-x86-pmu-ops.h header that can be used for the
> definition of static calls. This header is also intended to be
> used to simplify the defition of amd_pmu_ops and intel_pmu_ops.
> 
> Like what we did for kvm_x86_ops, 'pmu_ops' can be covered by
> static calls in a simlilar manner for insignificant but not
> negligible performance impact, especially on older models.
> 
> Signed-off-by: Like Xu <likexu@...cent.com>
> ---

This absolutely shouldn't be separated from patch 7/7.  By _defining_ the static
calls but not providing the logic to actually _update_ the calls, it's entirely
possible to add static_call() invocations that will compile cleanly without any
chance of doing the right thing at runtime.  

diff --git a/arch/x86/kvm/pmu.h b/arch/x86/kvm/pmu.h
index 0236c1a953d0..804f98b5552e 100644
--- a/arch/x86/kvm/pmu.h
+++ b/arch/x86/kvm/pmu.h
@@ -99,7 +99,7 @@ static inline bool pmc_is_fixed(struct kvm_pmc *pmc)

 static inline bool pmc_is_enabled(struct kvm_pmc *pmc)
 {
-       return kvm_pmu_ops.pmc_is_enabled(pmc);
+       return static_call(kvm_x86_pmu_pmc_is_enabled)(pmc);
 }

 static inline bool kvm_valid_perf_global_ctrl(struct kvm_pmu *pmu,

> @@ -0,0 +1,32 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#if !defined(KVM_X86_PMU_OP) || !defined(KVM_X86_PMU_OP_NULL)
> +BUILD_BUG_ON(1)
> +#endif
> +
> +/*
> + * KVM_X86_PMU_OP() and KVM_X86_PMU_OP_NULL() are used to

Please use all 80 chars.

> + * help generate "static_call()"s. They are also intended for use when defining
> + * the amd/intel KVM_X86_PMU_OPs. KVM_X86_PMU_OP() can be used

AMD/Intel since this is referring to the vendor and not to function names (like
the below reference).

> + * for those functions that follow the [amd|intel]_func_name convention.
> + * KVM_X86_PMU_OP_NULL() can leave a NULL definition for the

As below, please drop the _NULL() variant.

> + * case where there is no definition or a function name that
> + * doesn't match the typical naming convention is supplied.
> + */

...

> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> index 353989bf0102..bfdd9f2bc0fa 100644
> --- a/arch/x86/kvm/pmu.c
> +++ b/arch/x86/kvm/pmu.c
> @@ -50,6 +50,12 @@
>  struct kvm_pmu_ops kvm_pmu_ops __read_mostly;
>  EXPORT_SYMBOL_GPL(kvm_pmu_ops);
>  
> +#define	KVM_X86_PMU_OP(func)	\
> +	DEFINE_STATIC_CALL_NULL(kvm_x86_pmu_##func,	\
> +				*(((struct kvm_pmu_ops *)0)->func))
> +#define	KVM_X86_PMU_OP_NULL	KVM_X86_PMU_OP
> +#include <asm/kvm-x86-pmu-ops.h>
> +
>  static void kvm_pmi_trigger_fn(struct irq_work *irq_work)
>  {
>  	struct kvm_pmu *pmu = container_of(irq_work, struct kvm_pmu, irq_work);
> diff --git a/arch/x86/kvm/pmu.h b/arch/x86/kvm/pmu.h
> index b2fe135d395a..40e0b523637b 100644
> --- a/arch/x86/kvm/pmu.h
> +++ b/arch/x86/kvm/pmu.h
> @@ -45,6 +45,11 @@ struct kvm_pmu_ops {
>  	void (*cleanup)(struct kvm_vcpu *vcpu);
>  };
>  
> +#define	KVM_X86_PMU_OP(func)	\
> +	DECLARE_STATIC_CALL(kvm_x86_pmu_##func, *(((struct kvm_pmu_ops *)0)->func))
> +#define	KVM_X86_PMU_OP_NULL	KVM_X86_PMU_OP

I don't want to proliferate the pointless and bitrot-prone KVM_X86_OP_NULL macro,
just omit this.  I'll send a patch to drop KVM_X86_OP_NULL.

> +#include <asm/kvm-x86-pmu-ops.h>
> +
>  static inline u64 pmc_bitmask(struct kvm_pmc *pmc)
>  {
>  	struct kvm_pmu *pmu = pmc_to_pmu(pmc);
> -- 
> 2.33.0
>