Re: [PATCH v2 6/7] KVM: x86: Introduce definitions to support static calls for kvm_pmu_ops

[Like Xu]

Hi Sean,

On 9/12/2021 2:35 am, Sean Christopherson wrote:
> On Mon, Nov 08, 2021, Like Xu wrote:
> > From: Like Xu <likexu@xxxxxxxxxxx>
> >
> > 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@xxxxxxxxxxx>
> > ---
>
> 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