linux-kernel - Re: [PATCH 2/4] KVM: x86/pmu: Clear "reprogram" bit if counter is disabled or disallowed

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Message-ID: <2a83292b-a4d0-8d5e-b52a-31b7fcad2de6@gmail.com>
Date:   Fri, 14 Oct 2022 15:14:47 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Aaron Lewis <aaronlewis@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH 2/4] KVM: x86/pmu: Clear "reprogram" bit if counter is
 disabled or disallowed

For subject title, the "reprogram" bit is _only_ used to keep track of 
pmc->perf_event,
not whether the counter is disabled.

On 23/9/2022 8:13 am, Sean Christopherson wrote:
> When reprogramming a counter, clear the counter's "reprogram pending" bit
> if the counter is disabled (by the guest) or is disallowed (by the
> userspace filter).  In both cases, there's no need to re-attempt
> programming on the next coincident KVM_REQ_PMU as enabling the counter by
> either method will trigger reprogramming.

Perhaps we could move the check_pmu_event_filter() towards the top of the call 
stack.

> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>   arch/x86/kvm/pmu.c | 38 ++++++++++++++++++++++++--------------
>   1 file changed, 24 insertions(+), 14 deletions(-)
> 
> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> index 4504987cbbe2..4cd99320019b 100644
> --- a/arch/x86/kvm/pmu.c
> +++ b/arch/x86/kvm/pmu.c
> @@ -150,9 +150,9 @@ static void kvm_perf_overflow(struct perf_event *perf_event,
>   	__kvm_perf_overflow(pmc, true);
>   }
>   
> -static void pmc_reprogram_counter(struct kvm_pmc *pmc, u32 type,
> -				  u64 config, bool exclude_user,
> -				  bool exclude_kernel, bool intr)
> +static int pmc_reprogram_counter(struct kvm_pmc *pmc, u32 type, u64 config,
> +				 bool exclude_user, bool exclude_kernel,
> +				 bool intr)
>   {
>   	struct kvm_pmu *pmu = pmc_to_pmu(pmc);
>   	struct perf_event *event;
> @@ -204,14 +204,14 @@ static void pmc_reprogram_counter(struct kvm_pmc *pmc, u32 type,
>   	if (IS_ERR(event)) {
>   		pr_debug_ratelimited("kvm_pmu: event creation failed %ld for pmc->idx = %d\n",
>   			    PTR_ERR(event), pmc->idx);
> -		return;
> +		return PTR_ERR(event);
>   	}
>   
>   	pmc->perf_event = event;
>   	pmc_to_pmu(pmc)->event_count++;
> -	clear_bit(pmc->idx, pmc_to_pmu(pmc)->reprogram_pmi);
>   	pmc->is_paused = false;
>   	pmc->intr = intr || pebs;
> +	return 0;
>   }
>   
>   static void pmc_pause_counter(struct kvm_pmc *pmc)
> @@ -245,7 +245,6 @@ static bool pmc_resume_counter(struct kvm_pmc *pmc)
>   	perf_event_enable(pmc->perf_event);
>   	pmc->is_paused = false;
>   
> -	clear_bit(pmc->idx, (unsigned long *)&pmc_to_pmu(pmc)->reprogram_pmi);

This change is very suspicious.

>   	return true;
>   }
>   
> @@ -303,10 +302,10 @@ void reprogram_counter(struct kvm_pmc *pmc)
>   	pmc_pause_counter(pmc);
>   
>   	if (!pmc_speculative_in_use(pmc) || !pmc_is_enabled(pmc))
> -		return;
> +		goto reprogram_complete;
>   
>   	if (!check_pmu_event_filter(pmc))
> -		return;
> +		goto reprogram_complete;
>   
>   	if (eventsel & ARCH_PERFMON_EVENTSEL_PIN_CONTROL)
>   		printk_once("kvm pmu: pin control bit is ignored\n");
> @@ -324,16 +323,27 @@ void reprogram_counter(struct kvm_pmc *pmc)
>   	}
>   
>   	if (pmc->current_config == new_config && pmc_resume_counter(pmc))
> -		return;
> +		goto reprogram_complete;
>   
>   	pmc_release_perf_event(pmc);
>   
>   	pmc->current_config = new_config;
> -	pmc_reprogram_counter(pmc, PERF_TYPE_RAW,
> -			      (eventsel & pmu->raw_event_mask),
> -			      !(eventsel & ARCH_PERFMON_EVENTSEL_USR),
> -			      !(eventsel & ARCH_PERFMON_EVENTSEL_OS),
> -			      eventsel & ARCH_PERFMON_EVENTSEL_INT);
> +
> +	/*
> +	 * If reprogramming fails, e.g. due to contention, leave the counter's
> +	 * regprogram bit set, i.e. opportunistically try again on the next PMU

This is what we need, in the upstream case we need to keep trying regprogram
to try to occupy the hardware.

> +	 * refresh.  Don't make a new request as doing so can stall the guest
> +	 * if reprogramming repeatedly fails.

This does not happen, the guest still enters w/p perf_event backend support
and the vPMU is broken until the next vm-exit.

There is no need to endlessly call kvm_pmu_handle_event() when reprogram fails.

> +	 */
> +	if (pmc_reprogram_counter(pmc, PERF_TYPE_RAW,
> +				  (eventsel & pmu->raw_event_mask),
> +				  !(eventsel & ARCH_PERFMON_EVENTSEL_USR),
> +				  !(eventsel & ARCH_PERFMON_EVENTSEL_OS),
> +				  eventsel & ARCH_PERFMON_EVENTSEL_INT))
> +		return;
> +
> +reprogram_complete:
> +	clear_bit(pmc->idx, (unsigned long *)&pmc_to_pmu(pmc)->reprogram_pmi);
>   }
>   EXPORT_SYMBOL_GPL(reprogram_counter);
>