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

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Message-ID: <bfaac3e8-71a0-1e4a-0ff7-b25add6917d5@gmail.com>
Date:   Thu, 9 Dec 2021 20:50:12 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Sean Christopherson <seanjc@...gle.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

Hi Sean,

On 9/12/2021 2:35 am, Sean Christopherson wrote:
> 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.

Thanks for your clear comments on this patch set.

I will send out V3 once KVM_X86_OP_NULL is dropped as well as
the comment in arch/x86/include/asm/kvm-x86-ops.h is updated.

> 
>> +#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
>>
>