linux-kernel - Re: [PATCH v6 07/16] KVM: x86/pmu: Reprogram PEBS event to emulate guest PEBS counter

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Date:   Tue, 18 May 2021 15:36:06 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Xu, Like" <like.xu@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, weijiang.yang@...el.com,
        Kan Liang <kan.liang@...ux.intel.com>, ak@...ux.intel.com,
        wei.w.wang@...el.com, eranian@...gle.com, liuxiangdong5@...wei.com,
        linux-kernel@...r.kernel.org, x86@...nel.org, kvm@...r.kernel.org,
        Like Xu <like.xu@...ux.intel.com>
Subject: Re: [PATCH v6 07/16] KVM: x86/pmu: Reprogram PEBS event to emulate
 guest PEBS counter

On Tue, May 18, 2021 at 09:28:52PM +0800, Xu, Like wrote:

> > How would pebs && !intr be possible?
> 
> I don't think it's possible.

And yet you keep that 'intr||pebs' weirdness :/

> > Also; wouldn't this be more legible
> > when written like:
> > 
> > 	perf_overflow_handler_t ovf = kvm_perf_overflow;
> > 
> > 	...
> > 
> > 	if (intr)
> > 		ovf = kvm_perf_overflow_intr;
> > 
> > 	...
> > 
> > 	event = perf_event_create_kernel_counter(&attr, -1, current, ovf, pmc);
> > 
> 
> Please yell if you don't like this:
> 
> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> index 711294babb97..a607f5a1b9cd 100644
> --- a/arch/x86/kvm/pmu.c
> +++ b/arch/x86/kvm/pmu.c
> @@ -122,6 +122,8 @@ static void pmc_reprogram_counter(struct kvm_pmc *pmc,
> u32 type,
>                 .config = config,
>         };
>         bool pebs = test_bit(pmc->idx, (unsigned long *)&pmu->pebs_enable);
> +       perf_overflow_handler_t ovf = (intr || pebs) ?
> +               kvm_perf_overflow_intr : kvm_perf_overflow;

This, that's exactly the kind of code I wanted to get rid of. ?: has
it's place I suppose, but you're creating dense ugly code for no reason.

	perf_overflow_handle_t ovf = kvm_perf_overflow;

	if (intr)
		ovf = kvm_perf_overflow_intr;

Is so much easier to read. And if you really worry about that pebs
thing; you can add:

	WARN_ON_ONCE(pebs && !intr);