Re: [PATCH 5/5] KVM: x86: Add fastpath handling of HLT VM-Exits

[Manali Shukla]

Hi Sean,

On 8/3/2024 1:21 AM, Sean Christopherson wrote:
> 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.
> 

Could you please help me with the test case that resulted in an approximately
50% improvement for the nested scenario?

- Manali

> 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;