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);