Re: [PATCH v12 16/21] KVM: ARM64: Add PMU overflow interrupt routing

[Marc Zyngier <marc.zyngier@arm.com>]

On 22/02/16 09:37, Shannon Zhao wrote:
> From: Shannon Zhao <shannon.zhao@linaro.org>
> 
> When calling perf_event_create_kernel_counter to create perf_event,
> assign a overflow handler. Then when the perf event overflows, set the
> corresponding bit of guest PMOVSSET register. If this counter is enabled
> and its interrupt is enabled as well, kick the vcpu to sync the
> interrupt.
> 
> On VM entry, if there is counter overflowed, inject the interrupt with
> the level set to 1. Otherwise, inject the interrupt with the level set
> to 0.
> 
> Signed-off-by: Shannon Zhao <shannon.zhao@linaro.org>
> Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
> Reviewed-by: Andrew Jones <drjones@redhat.com>

As I mentioned yesterday, I was trying to pinpoint a performance drop, so I added PMU support to kvmtool (and made it an optional flag). This allowed be to find this:

> ---
>  arch/arm/kvm/arm.c    |  2 ++
>  include/kvm/arm_pmu.h |  2 ++
>  virt/kvm/arm/pmu.c    | 47 ++++++++++++++++++++++++++++++++++++++++++++++-
>  3 files changed, 50 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
> index dda1959..f54264c 100644
> --- a/arch/arm/kvm/arm.c
> +++ b/arch/arm/kvm/arm.c
> @@ -28,6 +28,7 @@
>  #include <linux/sched.h>
>  #include <linux/kvm.h>
>  #include <trace/events/kvm.h>
> +#include <kvm/arm_pmu.h>
>  
>  #define CREATE_TRACE_POINTS
>  #include "trace.h"
> @@ -577,6 +578,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
>  		 * non-preemptible context.
>  		 */
>  		preempt_disable();
> +		kvm_pmu_flush_hwstate(vcpu);
>  		kvm_timer_flush_hwstate(vcpu);
>  		kvm_vgic_flush_hwstate(vcpu);
>  
> diff --git a/include/kvm/arm_pmu.h b/include/kvm/arm_pmu.h
> index 8bc92d1..cf68f9a 100644
> --- a/include/kvm/arm_pmu.h
> +++ b/include/kvm/arm_pmu.h
> @@ -44,6 +44,7 @@ u64 kvm_pmu_valid_counter_mask(struct kvm_vcpu *vcpu);
>  void kvm_pmu_disable_counter(struct kvm_vcpu *vcpu, u64 val);
>  void kvm_pmu_enable_counter(struct kvm_vcpu *vcpu, u64 val);
>  void kvm_pmu_overflow_set(struct kvm_vcpu *vcpu, u64 val);
> +void kvm_pmu_flush_hwstate(struct kvm_vcpu *vcpu);
>  void kvm_pmu_software_increment(struct kvm_vcpu *vcpu, u64 val);
>  void kvm_pmu_handle_pmcr(struct kvm_vcpu *vcpu, u64 val);
>  void kvm_pmu_set_counter_event_type(struct kvm_vcpu *vcpu, u64 data,
> @@ -67,6 +68,7 @@ static inline u64 kvm_pmu_valid_counter_mask(struct kvm_vcpu *vcpu)
>  static inline void kvm_pmu_disable_counter(struct kvm_vcpu *vcpu, u64 val) {}
>  static inline void kvm_pmu_enable_counter(struct kvm_vcpu *vcpu, u64 val) {}
>  static inline void kvm_pmu_overflow_set(struct kvm_vcpu *vcpu, u64 val) {}
> +static inline void kvm_pmu_flush_hwstate(struct kvm_vcpu *vcpu) {}
>  static inline void kvm_pmu_software_increment(struct kvm_vcpu *vcpu, u64 val) {}
>  static inline void kvm_pmu_handle_pmcr(struct kvm_vcpu *vcpu, u64 val) {}
>  static inline void kvm_pmu_set_counter_event_type(struct kvm_vcpu *vcpu,
> diff --git a/virt/kvm/arm/pmu.c b/virt/kvm/arm/pmu.c
> index cda869c..6ac52ce 100644
> --- a/virt/kvm/arm/pmu.c
> +++ b/virt/kvm/arm/pmu.c
> @@ -21,6 +21,7 @@
>  #include <linux/perf_event.h>
>  #include <asm/kvm_emulate.h>
>  #include <kvm/arm_pmu.h>
> +#include <kvm/arm_vgic.h>
>  
>  /**
>   * kvm_pmu_get_counter_value - get PMU counter value
> @@ -181,6 +182,49 @@ void kvm_pmu_overflow_set(struct kvm_vcpu *vcpu, u64 val)
>  }
>  
>  /**
> + * kvm_pmu_flush_hwstate - flush pmu state to cpu
> + * @vcpu: The vcpu pointer
> + *
> + * Inject virtual PMU IRQ if IRQ is pending for this cpu.
> + */
> +void kvm_pmu_flush_hwstate(struct kvm_vcpu *vcpu)
> +{
> +	struct kvm_pmu *pmu = &vcpu->arch.pmu;
> +	u64 overflow;
> +
> +	if (!kvm_arm_pmu_v3_ready(vcpu))
> +		return;
> +
> +	overflow = kvm_pmu_overflow_status(vcpu);
> +	kvm_vgic_inject_irq(vcpu->kvm, vcpu->vcpu_id, pmu->irq_num, !!overflow);

It turns out that this single line costs us about 400 cycles on each entry:

maz@flakes:~/kvm-ws-tests$ make LKVM=~/kvmtool/lkvm LKVM_ARGS=--pmu PERF=perf_4.3 tests-gicv2
GICv2:
do_hvc.bin:5690.17
do_sgi.bin:9395.05
do_sysreg.bin:5912.6
maz@flakes:~/kvm-ws-tests$ make LKVM=~/kvmtool/lkvm PERF=perf_4.3 tests-gicv2
GICv2:
do_hvc.bin:5285.02
do_sgi.bin:9131.24
do_sysreg.bin:5563.7

Caching the irq state and only injecting it if it has changed (just like we do for the timer) brings performance back to its previous level:

diff --git a/include/kvm/arm_pmu.h b/include/kvm/arm_pmu.h
index 176913f..b23e636 100644
--- a/include/kvm/arm_pmu.h
+++ b/include/kvm/arm_pmu.h
@@ -35,6 +35,7 @@ struct kvm_pmu {
 	int irq_num;
 	struct kvm_pmc pmc[ARMV8_PMU_MAX_COUNTERS];
 	bool ready;
+	bool irq_level;
 };
 
 #define kvm_arm_pmu_v3_ready(v)		((v)->arch.pmu.ready)
diff --git a/virt/kvm/arm/pmu.c b/virt/kvm/arm/pmu.c
index 904617e..7156f8b 100644
--- a/virt/kvm/arm/pmu.c
+++ b/virt/kvm/arm/pmu.c
@@ -229,13 +229,17 @@ void kvm_pmu_overflow_set(struct kvm_vcpu *vcpu, u64 val)
 void kvm_pmu_flush_hwstate(struct kvm_vcpu *vcpu)
 {
 	struct kvm_pmu *pmu = &vcpu->arch.pmu;
-	u64 overflow;
+	bool overflow;
 
 	if (!kvm_arm_pmu_v3_ready(vcpu))
 		return;
 
-	overflow = kvm_pmu_overflow_status(vcpu);
-	kvm_vgic_inject_irq(vcpu->kvm, vcpu->vcpu_id, pmu->irq_num, !!overflow);
+	overflow = !!kvm_pmu_overflow_status(vcpu);
+	if (pmu->irq_level != overflow) {
+		pmu->irq_level = overflow;
+		kvm_vgic_inject_irq(vcpu->kvm, vcpu->vcpu_id,
+				    pmu->irq_num, overflow);
+	}
 }
 
 static inline struct kvm_vcpu *kvm_pmc_to_vcpu(struct kvm_pmc *pmc)


Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...