Re: [PATCH] kvm: x86: move srcu lock out of kvm_vcpu_check_block

[Paolo Bonzini <pbonzini@redhat.com>]

On 30/04/21 22:45, Sean Christopherson wrote:
> On Wed, Apr 28, 2021, Jon Kohler wrote:
>> To improve performance, this moves kvm->srcu lock logic from
>> kvm_vcpu_check_block to kvm_vcpu_running and wraps directly around
>> check_events. Also adds a hint for callers to tell
>> kvm_vcpu_running whether or not to acquire srcu, which is useful in
>> situations where the lock may already be held. With this in place, we
>> see roughly 5% improvement in an internal benchmark [3] and no more
>> impact from this lock on non-nested workloads.
> 
> ...
> 
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index efc7a82ab140..354f690cc982 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -9273,10 +9273,24 @@ static inline int vcpu_block(struct kvm *kvm, struct kvm_vcpu *vcpu)
>>   	return 1;
>>   }
>>
>> -static inline bool kvm_vcpu_running(struct kvm_vcpu *vcpu)
>> +static inline bool kvm_vcpu_running(struct kvm_vcpu *vcpu, bool acquire_srcu)
>>   {
>> -	if (is_guest_mode(vcpu))
>> -		kvm_x86_ops.nested_ops->check_events(vcpu);
>> +	if (is_guest_mode(vcpu)) {
>> +		if (acquire_srcu) {
>> +			/*
>> +			 * We need to lock because check_events could call
>> +			 * nested_vmx_vmexit() which might need to resolve a
>> +			 * valid memslot. We will have this lock only when
>> +			 * called from vcpu_run but not when called from
>> +			 * kvm_vcpu_check_block > kvm_arch_vcpu_runnable.
>> +			 */
>> +			int idx = srcu_read_lock(&vcpu->kvm->srcu);
>> +			kvm_x86_ops.nested_ops->check_events(vcpu);
>> +			srcu_read_unlock(&vcpu->kvm->srcu, idx);
>> +		} else {
>> +			kvm_x86_ops.nested_ops->check_events(vcpu);
>> +		}
>> +	}
> 
> Obviously not your fault, but I absolutely detest calling check_events() from
> kvm_vcpu_running.  I would much prefer to make baby steps toward cleaning up the
> existing mess instead of piling more weirdness on top.
> 
> Ideally, APICv support would be fixed to not require a deep probe into nested
> events just to see if a vCPU can run.  But, that's probably more than we want to
> bite off at this time.
> 
> What if we add another nested_ops API to check if the vCPU has an event, but not
> actually process the event?  I think that would allow eliminating the SRCU lock,
> and would get rid of the most egregious behavior of triggering a nested VM-Exit
> in a seemingly innocuous helper.
> 
> If this works, we could even explore moving the call to nested_ops->has_events()
> out of kvm_vcpu_running() and into kvm_vcpu_has_events(); I can't tell if the
> side effects in vcpu_block() would get messed up with that change :-/
> 
> Incomplete patch...

I think it doesn't even have to be *nested* events.  Most events are the 
same inside or outside guest mode, as they already special case guest 
mode inside the kvm_x86_ops callbacks (e.g. kvm_arch_interrupt_allowed 
is already called by kvm_vcpu_has_events).

I think we only need to extend kvm_x86_ops.nested_ops->hv_timer_pending 
to cover MTF, plus double check that INIT and SIPI are handled 
correctly, and then the call to check_nested_events can go away.

Paolo

> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 00339d624c92..15f514891326 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -3771,15 +3771,17 @@ static bool nested_vmx_preemption_timer_pending(struct kvm_vcpu *vcpu)
>                 to_vmx(vcpu)->nested.preemption_timer_expired;
>   }
> 
> -static int vmx_check_nested_events(struct kvm_vcpu *vcpu)
> +static int __vmx_check_nested_events(struct kvm_vcpu *vcpu, bool only_check)
>   {
>          struct vcpu_vmx *vmx = to_vmx(vcpu);
>          unsigned long exit_qual;
> -       bool block_nested_events =
> -           vmx->nested.nested_run_pending || kvm_event_needs_reinjection(vcpu);
>          bool mtf_pending = vmx->nested.mtf_pending;
>          struct kvm_lapic *apic = vcpu->arch.apic;
> 
> +       bool block_nested_events = only_check ||
> +                                  vmx->nested.nested_run_pending ||
> +                                  kvm_event_needs_reinjection(vcpu);
> +
>          /*
>           * Clear the MTF state. If a higher priority VM-exit is delivered first,
>           * this state is discarded.
> @@ -3837,7 +3839,7 @@ static int vmx_check_nested_events(struct kvm_vcpu *vcpu)
>          }
> 
>          if (vcpu->arch.exception.pending) {
> -               if (vmx->nested.nested_run_pending)
> +               if (vmx->nested.nested_run_pending || only_check)
>                          return -EBUSY;
>                  if (!nested_vmx_check_exception(vcpu, &exit_qual))
>                          goto no_vmexit;
> @@ -3886,10 +3888,23 @@ static int vmx_check_nested_events(struct kvm_vcpu *vcpu)
>          }
> 
>   no_vmexit:
> -       vmx_complete_nested_posted_interrupt(vcpu);
> +       if (!check_only)
> +               vmx_complete_nested_posted_interrupt(vcpu);
> +       else if (vmx->nested.pi_desc && vmx->nested.pi_pending)
> +               return -EBUSY;
>          return 0;
>   }
> 
> +static bool vmx_has_nested_event(struct kvm_vcpu *vcpu)
> +{
> +       return !!__vmx_check_nested_events(vcpu, true);
> +}
> +
> +static int vmx_check_nested_events(struct kvm_vcpu *vcpu)
> +{
> +       return __vmx_check_nested_events(vcpu, false);
> +}
> +
>   static u32 vmx_get_preemption_timer_value(struct kvm_vcpu *vcpu)
>   {
>          ktime_t remaining =
> @@ -6627,6 +6642,7 @@ __init int nested_vmx_hardware_setup(int (*exit_handlers[])(struct kvm_vcpu *))
>   }
> 
>   struct kvm_x86_nested_ops vmx_nested_ops = {
> +       .has_event = vmx_has_nested_event,
>          .check_events = vmx_check_nested_events,
>          .hv_timer_pending = nested_vmx_preemption_timer_pending,
>          .triple_fault = nested_vmx_triple_fault,
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index a829f1ab60c3..5df01012cb1f 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9310,6 +9310,10 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>                          update_cr8_intercept(vcpu);
>                          kvm_lapic_sync_to_vapic(vcpu);
>                  }
> +       } else if (is_guest_mode(vcpu)) {
> +               r = kvm_check_nested_events(vcpu);
> +               if (r < 0)
> +                       req_immediate_exit = true;
>          }
> 
>          r = kvm_mmu_reload(vcpu);
> @@ -9516,8 +9520,10 @@ static inline int vcpu_block(struct kvm *kvm, struct kvm_vcpu *vcpu)
> 
>   static inline bool kvm_vcpu_running(struct kvm_vcpu *vcpu)
>   {
> -       if (is_guest_mode(vcpu))
> -               kvm_check_nested_events(vcpu);
> +       if (is_guest_mode(vcpu) &&
> +           (kvm_test_request(KVM_REQ_TRIPLE_FAULT, vcpu) ||
> +            kvm_x86_ops.nested_ops->has_event(vcpu)))
> +               return true;
> 
>          return (vcpu->arch.mp_state == KVM_MP_STATE_RUNNABLE &&
>                  !vcpu->arch.apf.halted);
>