Re: [PATCH v2 4/4] KVM: x86/vPMU: Add lazy mechanism to release perf_event per vPMC

[Like Xu <like.xu@linux.intel.com>]

Hi Paolo,

On 2019/10/21 16:45, Paolo Bonzini wrote:
> On 13/10/19 11:15, Like Xu wrote:
>> Currently, a host perf_event is created for a vPMC functionality emulation.
>> It’s unpredictable to determine if a disabled perf_event will be reused.
>> If they are disabled and are not reused for a considerable period of time,
>> those obsolete perf_events would increase host context switch overhead that
>> could have been avoided.
>>
>> If the guest doesn't access (set_msr/get_msr/rdpmc) any of the vPMC's MSRs
>> during an entire vcpu sched time slice, and its independent enable bit of
>> the vPMC isn't set, we can predict that the guest has finished the use of
>> this vPMC, and then it's time to release non-reused perf_events in the
>> first call of vcpu_enter_guest() after the vcpu gets next scheduled in.
>>
>> This lazy mechanism delays the event release time to the beginning of the
>> next scheduled time slice if vPMC's MSRs aren't accessed during this time
>> slice. If guest comes back to use this vPMC in next time slice, a new perf
>> event would be re-created via perf_event_create_kernel_counter() as usual.
>>
>> Suggested-by: Wei W Wang <wei.w.wang@intel.com>
>> Signed-off-by: Like Xu <like.xu@linux.intel.com>
>> ---
>>   arch/x86/include/asm/kvm_host.h | 15 ++++++++++++
>>   arch/x86/kvm/pmu.c              | 43 +++++++++++++++++++++++++++++++++
>>   arch/x86/kvm/pmu.h              |  3 +++
>>   arch/x86/kvm/pmu_amd.c          | 13 ++++++++++
>>   arch/x86/kvm/vmx/pmu_intel.c    | 25 +++++++++++++++++++
>>   arch/x86/kvm/x86.c              | 12 +++++++++
>>   6 files changed, 111 insertions(+)
>>
>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>> index 1abbbbae4953..45f9cdae150b 100644
>> --- a/arch/x86/include/asm/kvm_host.h
>> +++ b/arch/x86/include/asm/kvm_host.h
>> @@ -472,6 +472,21 @@ struct kvm_pmu {
>>   	struct kvm_pmc fixed_counters[INTEL_PMC_MAX_FIXED];
>>   	struct irq_work irq_work;
>>   	u64 reprogram_pmi;
>> +
>> +	/* for vPMC being set, do not released its perf_event (if any) */
>> +	u64 lazy_release_ctrl;
> 
> Please use DECLARE_BITMAP(lazy_release_ctrl, X86_PMC_IDX_MAX).  I would
> also rename the bitmap to pmc_in_use.

Thanks for introducing BITMAP and I would definitely use it if I knew.

> 
> I know you're just copying what reprogram_pmi does, but that has to be
> fixed too. :)  I'll send a patch now.
> 
>> +	/*
>> +	 * The gate to release perf_events not marked in
>> +	 * lazy_release_ctrl only once in a vcpu time slice.
>> +	 */
>> +	bool need_cleanup;
>> +
>> +	/*
>> +	 * The total number of programmed perf_events and it helps to avoid
>> +	 * redundant check before cleanup if guest don't use vPMU at all.
>> +	 */
>> +	u8 event_count;
>>   };
>>   
>>   struct kvm_pmu_ops;
>> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
>> index 09d1a03c057c..7ab262f009f6 100644
>> --- a/arch/x86/kvm/pmu.c
>> +++ b/arch/x86/kvm/pmu.c
>> @@ -137,6 +137,7 @@ static void pmc_reprogram_counter(struct kvm_pmc *pmc, u32 type,
>>   	}
>>   
>>   	pmc->perf_event = event;
>> +	pmc_to_pmu(pmc)->event_count++;
>>   	clear_bit(pmc->idx, (unsigned long*)&pmc_to_pmu(pmc)->reprogram_pmi);
>>   }
>>   
>> @@ -368,6 +369,7 @@ int kvm_pmu_rdpmc(struct kvm_vcpu *vcpu, unsigned idx, u64 *data)
>>   	if (!pmc)
>>   		return 1;
>>   
>> +	__set_bit(pmc->idx, (unsigned long *)&pmu->lazy_release_ctrl);
>>   	*data = pmc_read_counter(pmc) & mask;
>>   	return 0;
>>   }
>> @@ -385,11 +387,13 @@ bool kvm_pmu_is_valid_msr(struct kvm_vcpu *vcpu, u32 msr)
>>   
>>   int kvm_pmu_get_msr(struct kvm_vcpu *vcpu, u32 msr, u64 *data)
>>   {
>> +	kvm_x86_ops->pmu_ops->update_lazy_release_ctrl(vcpu, msr);
> 
> Instead of this new pmu_ops entry, please introduce two separate patches
> to do the following:
> 
> 1) rename the existing msr_idx_to_pmc to rdpmc_idx_to_pmc, and
> is_valid_msr_idx to is_valid_rdpmc_idx (likewise for
> kvm_pmu_is_valid_msr_idx).

It looks good to me and I'll apply it.

> 
> 2) introduce a new callback msr_idx_to_pmc that returns a struct
> kvm_pmc*, and change kvm_pmu_is_valid_msr to do

For callback msr_idx_to_pmc,
how do we deal with the input 'MSR_CORE_PERF_FIXED_CTR_CTRL'
which may return several fixed kvm_pmcs not just one?

> 
> bool kvm_pmu_is_valid_msr(struct kvm_vcpu *vcpu, u32 msr)
> {
>          return (kvm_x86_ops->pmu_ops->msr_idx_to_pmc(vcpu, msr) ||
> 		kvm_x86_ops->pmu_ops->is_valid_msr(vcpu, msr));
> }
> 
> and AMD can just return false from .is_valid_msr.
> 
> Once this change is done, this patch can use simply:
> 
> static int kvm_pmu_mark_pmc_in_use(struct kvm_vcpu *vcpu, u32 msr)

s/static int/static void/.

> {
> 	struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
> 	struct kvm_pmc *pmc = kvm_x86_ops->pmu_ops->msr_idx_to_pmc(vcpu, msr);

We need 'if(pmc)' here.

> 	__set_bit(pmc->idx, pmu->pmc_in_use);
> }
> 
>>   	return kvm_x86_ops->pmu_ops->get_msr(vcpu, msr, data);
>>   }
>>   
>>   int kvm_pmu_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>>   {
>> +	kvm_x86_ops->pmu_ops->update_lazy_release_ctrl(vcpu, msr_info->index);
>>   	return kvm_x86_ops->pmu_ops->set_msr(vcpu, msr_info);
>>   }
>>   
>> @@ -417,9 +421,48 @@ void kvm_pmu_init(struct kvm_vcpu *vcpu)
>>   	memset(pmu, 0, sizeof(*pmu));
>>   	kvm_x86_ops->pmu_ops->init(vcpu);
>>   	init_irq_work(&pmu->irq_work, kvm_pmi_trigger_fn);
>> +	pmu->lazy_release_ctrl = 0;
>> +	pmu->event_count = 0;
>> +	pmu->need_cleanup = false;
>>   	kvm_pmu_refresh(vcpu);
>>   }
>>   
>> +static inline bool pmc_speculative_in_use(struct kvm_pmc *pmc)
>> +{
>> +	struct kvm_pmu *pmu = pmc_to_pmu(pmc);
>> +
>> +	if (pmc_is_fixed(pmc))
>> +		return fixed_ctrl_field(pmu->fixed_ctr_ctrl,
>> +			pmc->idx - INTEL_PMC_IDX_FIXED) & 0x3;
>> +
>> +	return pmc->eventsel & ARCH_PERFMON_EVENTSEL_ENABLE;
>> +}
>> +
>> +void kvm_pmu_cleanup(struct kvm_vcpu *vcpu)
>> +{
>> +	struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
>> +	struct kvm_pmc *pmc = NULL;
>> +	u64 bitmask = ~pmu->lazy_release_ctrl;
> 
> DECLARE_BITMAP(bitmask, X86_PMC_IDX_MAX);
> 
>> +	int i;
>> +
>> +	if (!unlikely(pmu->need_cleanup))
>> +		return;
>> +
>> +	/* do cleanup before the first time of running vcpu after sched_in */
>> +	pmu->need_cleanup = false;
>> +
>> +	/* release events for unmarked vPMCs in the last sched time slice */
>> +	for_each_set_bit(i, (unsigned long *)&bitmask, X86_PMC_IDX_MAX) {
> 
> First, you could use for_each_clear_bit instead of inverting.
> 
> However, this is iterating unnecessarily through almost all of the 64
> bits, most of which will not pass pmc_idx_to_pmc.  You can set up a
> bitmap like

Nice catch and let's trade space for time.

> 
> 	/* in kvm_pmu_refresh */
> 	bitmap_set(pmu->all_valid_pmc_idx, 0,
> 		   pmu->nr_arch_fixed_counters);
> 	bitmap_set(pmu->all_valid_pmc_idx, INTEL_PMC_IDX_FIXED,
> 		   pmu->nr_arch_gp_counters);
> 
> 	/* on cleanup */
> 	bitmap_andnot(bitmask, pmu->all_valid_pmc_idx,
> 		      pmu->pmc_in_use, X86_PMC_IDX_MAX);
> 
> 	for_each_set_bit(i, bitmask, X86_PMC_IDX_MAX) {
> 		...
> 	}
> 
> Note that on 64-bit machines the bitmap_andnot will be compiled to a
> single "x = y & ~z" expression.

Thanks for your tutorial on bitmap APIs, again.

> 
>> +		pmc = kvm_x86_ops->pmu_ops->pmc_idx_to_pmc(pmu, i);
>> +
>> +		if (pmc && pmc->perf_event && !pmc_speculative_in_use(pmc))
>> +			pmc_stop_counter(pmc);
>> +	}
>> +
>> +	/* reset vPMC lazy-release bitmap for this sched time slice */
>> +	pmu->lazy_release_ctrl = 0;
>> +}
>> +
>>   void kvm_pmu_destroy(struct kvm_vcpu *vcpu)
>>   {
>>   	kvm_pmu_reset(vcpu);
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 661e2bf38526..023ea5efb3bb 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -8080,6 +8080,14 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>>   		goto cancel_injection;
>>   	}
>>   
>> +	/*
>> +	 * vPMU uses a lazy method to release the perf_events created for
>> +	 * features emulation when the related MSRs weren't accessed during
>> +	 * last vcpu time slice. Technically, this cleanup check happens on
>> +	 * the first call of vcpu_enter_guest after the vcpu gets scheduled in.
>> +	 */
>> +	kvm_pmu_cleanup(vcpu);
> 
> Instead of calling this unconditionally from vcpu_enter_guest, please
> request KVM_REQ_PMU in kvm_arch_sched_in, and call kvm_pmu_cleanup from
> kvm_pmu_handle_event.

This proposal does make sense to the name 'kvm_pmu_handle_event' and 
I'll apply it.

> 
> Paolo
> 
>>   	preempt_disable();
>>   
>>   	kvm_x86_ops->prepare_guest_switch(vcpu);
>> @@ -9415,7 +9423,11 @@ void kvm_arch_vcpu_uninit(struct kvm_vcpu *vcpu)
>>   
>>   void kvm_arch_sched_in(struct kvm_vcpu *vcpu, int cpu)
>>   {
>> +	struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
>> +
>>   	vcpu->arch.l1tf_flush_l1d = true;
>> +	if (pmu->version && unlikely(pmu->event_count))
>> +		pmu->need_cleanup = true;
>>   	kvm_x86_ops->sched_in(vcpu, cpu);
>>   }
>>   
>>
> 
>