[PATCH] KVM: arm64: pmu: Resync EL0 state on counter rotation

[Shijie Huang]

Hi Marc,

在 2023/8/12 2:05, Marc Zyngier 写道:
> Huang Shijie reports that, when profiling a guest from the host
> with a number of events that exceeds the number of available
> counters, the reported counts are wildly inaccurate. Without
> the counter oversubscription, the reported counts are correct.
>
> Their investigation indicates that upon counter rotation (which
> takes place on the back of a timer interrupt), we fail to
> re-apply the guest EL0 enabling, leading to the counting of host
> events instead of guest events.
>
> In order to solve this, add yet another hook between the host PMU
> driver and KVM, re-applying the guest EL0 configuration if the
> right conditions apply (the host is VHE, we are in interrupt
> context, and we interrupted a running vcpu). This triggers a new
> vcpu request which will apply the correct configuration on guest
> reentry.
>
> With this, we have the correct counts, even when the counters are
> oversubscribed.
>
> Reported-by: Huang Shijie <shijie at os.amperecomputing.com>
> Suggested-by: Oliver Upton <oliver.upton at linux.dev>
> Signed-off-by: Marc Zyngier <maz at kernel.org>
> Cc: Mark Rutland <mark.rutland at arm.com>
> Cc: Will Deacon <will at kernel.org>
> Link: https://lore.kernel.org/r/20230809013953.7692-1-shijie@os.amperecomputing.com
> ---
>   arch/arm64/include/asm/kvm_host.h |  1 +
>   arch/arm64/kvm/arm.c              |  3 +++
>   arch/arm64/kvm/pmu.c              | 18 ++++++++++++++++++
>   drivers/perf/arm_pmuv3.c          |  2 ++
>   include/kvm/arm_pmu.h             |  2 ++
>   5 files changed, 26 insertions(+)
>
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index d3dd05bbfe23..553040e0e375 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -49,6 +49,7 @@
>   #define KVM_REQ_RELOAD_GICv4	KVM_ARCH_REQ(4)
>   #define KVM_REQ_RELOAD_PMU	KVM_ARCH_REQ(5)
>   #define KVM_REQ_SUSPEND		KVM_ARCH_REQ(6)
> +#define KVM_REQ_RESYNC_PMU_EL0	KVM_ARCH_REQ(7)
>   
>   #define KVM_DIRTY_LOG_MANUAL_CAPS   (KVM_DIRTY_LOG_MANUAL_PROTECT_ENABLE | \
>   				     KVM_DIRTY_LOG_INITIALLY_SET)
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index 72dc53a75d1c..978b0411082f 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -803,6 +803,9 @@ static int check_vcpu_requests(struct kvm_vcpu *vcpu)
>   			kvm_pmu_handle_pmcr(vcpu,
>   					    __vcpu_sys_reg(vcpu, PMCR_EL0));
>   
> +		if (kvm_check_request(KVM_REQ_RESYNC_PMU_EL0, vcpu))
> +			kvm_vcpu_pmu_restore_guest(vcpu);
> +
>   		if (kvm_check_request(KVM_REQ_SUSPEND, vcpu))
>   			return kvm_vcpu_suspend(vcpu);
>   
> diff --git a/arch/arm64/kvm/pmu.c b/arch/arm64/kvm/pmu.c
> index 121f1a14c829..0eea225fd09a 100644
> --- a/arch/arm64/kvm/pmu.c
> +++ b/arch/arm64/kvm/pmu.c
> @@ -236,3 +236,21 @@ bool kvm_set_pmuserenr(u64 val)
>   	ctxt_sys_reg(hctxt, PMUSERENR_EL0) = val;
>   	return true;
>   }
> +
> +/*
> + * If we interrupted the guest to update the host PMU context, make
> + * sure we re-apply the guest EL0 state.
> + */
> +void kvm_vcpu_pmu_resync_el0(void)
> +{
> +	struct kvm_vcpu *vcpu;
> +
> +	if (!has_vhe() || !in_interrupt())
> +		return;
> +
> +	vcpu = kvm_get_running_vcpu();
> +	if (!vcpu)
> +		return;
> +
> +	kvm_make_request(KVM_REQ_RESYNC_PMU_EL0, vcpu);
> +}
> diff --git a/drivers/perf/arm_pmuv3.c b/drivers/perf/arm_pmuv3.c
> index 08b3a1bf0ef6..6a3d8176f54a 100644
> --- a/drivers/perf/arm_pmuv3.c
> +++ b/drivers/perf/arm_pmuv3.c
> @@ -772,6 +772,8 @@ static void armv8pmu_start(struct arm_pmu *cpu_pmu)
>   
>   	/* Enable all counters */
>   	armv8pmu_pmcr_write(armv8pmu_pmcr_read() | ARMV8_PMU_PMCR_E);
> +
> +	kvm_vcpu_pmu_resync_el0();
>   }

I read the perf code again, and find it maybe not good to do it in 
armv8pmu_start.

    Assume we install a new perf event to a CPU "x" from CPU 0,  a VM 
guest is running on CPU "x":

     perf_event_open() --> perf_install_in_context() -->

     call this function in  IPI interrupt: ___perf_install_in_context().

    armv8pmu_start() will be called in ___perf_install_in_context() in IPI.

    so kvm_vcpu_pmu_resync_el0() will _make_ a request by meeting the 
conditions:

              1.) in interrupt context.

              2.) a guest is running on this CPU.


But in actually, the request should not send out.

Please correct me if I am wrong.

IMHO, the best solution is add  a hook in the perf/core code, and make 
the request there.

I will send my v3 patch.


Thanks

Huang Shijie






>   
>   static void armv8pmu_stop(struct arm_pmu *cpu_pmu)
> diff --git a/include/kvm/arm_pmu.h b/include/kvm/arm_pmu.h
> index 847da6fc2713..3a8a70a60794 100644
> --- a/include/kvm/arm_pmu.h
> +++ b/include/kvm/arm_pmu.h
> @@ -74,6 +74,7 @@ int kvm_arm_pmu_v3_enable(struct kvm_vcpu *vcpu);
>   struct kvm_pmu_events *kvm_get_pmu_events(void);
>   void kvm_vcpu_pmu_restore_guest(struct kvm_vcpu *vcpu);
>   void kvm_vcpu_pmu_restore_host(struct kvm_vcpu *vcpu);
> +void kvm_vcpu_pmu_resync_el0(void);
>   
>   #define kvm_vcpu_has_pmu(vcpu)					\
>   	(test_bit(KVM_ARM_VCPU_PMU_V3, (vcpu)->arch.features))
> @@ -171,6 +172,7 @@ static inline u8 kvm_arm_pmu_get_pmuver_limit(void)
>   {
>   	return 0;
>   }
> +static inline void kvm_vcpu_pmu_resync_el0(void) {}
>   
>   #endif
>