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

[Sean Christopherson]

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...

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);