Add a fastpath for HLT VM-Exits by immediately re-entering the guest if it has a pending wake event. When virtual interrupt delivery is enabled, i.e. when KVM doesn't need to manually inject interrupts, this allows KVM to stay in the fastpath run loop when a vIRQ arrives between the guest doing CLI and STI;HLT. Without AMD's Idle HLT-intercept support, the CPU generates a HLT VM-Exit even though KVM will immediately resume the guest. Note, on bare metal, it's relatively uncommon for a modern guest kernel to actually trigger this scenario, as the window between the guest checking for a wake event and committing to HLT is quite small. But in a nested environment, the timings change significantly, e.g. rudimentary testing showed that ~50% of HLT exits where HLT-polling was successful would be serviced by this fastpath, i.e. ~50% of the time that a nested vCPU gets a wake event before KVM schedules out the vCPU, the wake event was pending even before the VM-Exit. Link: https://lore.kernel.org/all/20240528041926.3989-3-manali.shukla@xxxxxxx Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx> --- arch/x86/kvm/svm/svm.c | 13 +++++++++++-- arch/x86/kvm/vmx/vmx.c | 2 ++ arch/x86/kvm/x86.c | 23 ++++++++++++++++++++++- arch/x86/kvm/x86.h | 1 + 4 files changed, 36 insertions(+), 3 deletions(-) diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c index c115d26844f7..64381ff63034 100644 --- a/arch/x86/kvm/svm/svm.c +++ b/arch/x86/kvm/svm/svm.c @@ -4144,12 +4144,21 @@ static int svm_vcpu_pre_run(struct kvm_vcpu *vcpu) static fastpath_t svm_exit_handlers_fastpath(struct kvm_vcpu *vcpu) { + struct vcpu_svm *svm = to_svm(vcpu); + if (is_guest_mode(vcpu)) return EXIT_FASTPATH_NONE; - if (to_svm(vcpu)->vmcb->control.exit_code == SVM_EXIT_MSR && - to_svm(vcpu)->vmcb->control.exit_info_1) + switch (svm->vmcb->control.exit_code) { + case SVM_EXIT_MSR: + if (!svm->vmcb->control.exit_info_1) + break; return handle_fastpath_set_msr_irqoff(vcpu); + case SVM_EXIT_HLT: + return handle_fastpath_hlt(vcpu); + default: + break; + } return EXIT_FASTPATH_NONE; } diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c index f18c2d8c7476..f6382750fbf0 100644 --- a/arch/x86/kvm/vmx/vmx.c +++ b/arch/x86/kvm/vmx/vmx.c @@ -7265,6 +7265,8 @@ static fastpath_t vmx_exit_handlers_fastpath(struct kvm_vcpu *vcpu, return handle_fastpath_set_msr_irqoff(vcpu); case EXIT_REASON_PREEMPTION_TIMER: return handle_fastpath_preemption_timer(vcpu, force_immediate_exit); + case EXIT_REASON_HLT: + return handle_fastpath_hlt(vcpu); default: return EXIT_FASTPATH_NONE; } diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 46686504cd47..eb5ea963698f 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -11373,7 +11373,10 @@ static int __kvm_emulate_halt(struct kvm_vcpu *vcpu, int state, int reason) */ ++vcpu->stat.halt_exits; if (lapic_in_kernel(vcpu)) { - vcpu->arch.mp_state = state; + if (kvm_vcpu_has_events(vcpu)) + vcpu->arch.pv.pv_unhalted = false; + else + vcpu->arch.mp_state = state; return 1; } else { vcpu->run->exit_reason = reason; @@ -11398,6 +11401,24 @@ int kvm_emulate_halt(struct kvm_vcpu *vcpu) } EXPORT_SYMBOL_GPL(kvm_emulate_halt); +fastpath_t handle_fastpath_hlt(struct kvm_vcpu *vcpu) +{ + int ret; + + kvm_vcpu_srcu_read_lock(vcpu); + ret = kvm_emulate_halt(vcpu); + kvm_vcpu_srcu_read_unlock(vcpu); + + if (!ret) + return EXIT_FASTPATH_EXIT_USERSPACE; + + if (kvm_vcpu_running(vcpu)) + return EXIT_FASTPATH_REENTER_GUEST; + + return EXIT_FASTPATH_EXIT_HANDLED; +} +EXPORT_SYMBOL_GPL(handle_fastpath_hlt); + int kvm_emulate_ap_reset_hold(struct kvm_vcpu *vcpu) { int ret = kvm_skip_emulated_instruction(vcpu); diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h index 50596f6f8320..5185ab76fdd2 100644 --- a/arch/x86/kvm/x86.h +++ b/arch/x86/kvm/x86.h @@ -334,6 +334,7 @@ int x86_decode_emulated_instruction(struct kvm_vcpu *vcpu, int emulation_type, int x86_emulate_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, int emulation_type, void *insn, int insn_len); fastpath_t handle_fastpath_set_msr_irqoff(struct kvm_vcpu *vcpu); +fastpath_t handle_fastpath_hlt(struct kvm_vcpu *vcpu); extern struct kvm_caps kvm_caps; extern struct kvm_host_values kvm_host; -- 2.46.0.rc2.264.g509ed76dc8-goog
