Re: [PATCH 2/3] KVM: Add capability to not exit on HLT

["Michael S. Tsirkin" <mst@redhat.com>]

On Sat, Nov 25, 2017 at 02:09:32PM +0100, Jan H. Schönherr wrote:
> If host CPUs are dedicated to a VM, we can avoid VM exits on HLT,
> reducing the wake-up latency on posted interrupts.
> 
> This reintroduces a feature that has been there at some point --
> see Linux 3.4 commit 10166744b80a ("KVM: VMX: remove yield_on_hlt")
> for the removal -- but with the additional ability to enable it only
> for selected VMs (and supporting SVM as well).
> 
> Signed-off-by: Jan H. Schönherr <jschoenh@amazon.de>


If you are going to do this, why not expose mwait
in the cpuid thus making guests use mwait to halt?

What are the advantages of doing this using halt specifically?



> ---
> Note: AMD code paths are only compile tested
> ---
>  Documentation/virtual/kvm/api.txt | 12 +++++++++++-
>  arch/x86/include/asm/kvm_host.h   |  1 +
>  arch/x86/kvm/svm.c                |  3 ++-
>  arch/x86/kvm/vmx.c                | 33 +++++++++++++++++++++++++++------
>  arch/x86/kvm/x86.c                |  5 +++++
>  arch/x86/kvm/x86.h                |  5 +++++
>  include/uapi/linux/kvm.h          |  1 +
>  7 files changed, 52 insertions(+), 8 deletions(-)
> 
> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> index 0ee812c..c06bb41 100644
> --- a/Documentation/virtual/kvm/api.txt
> +++ b/Documentation/virtual/kvm/api.txt
> @@ -4172,7 +4172,17 @@ Returns: 0 on success
>  This capability indicates that a guest using memory monitoring instructions
>  (MWAIT/MWAITX) to stop a virtual CPU will not cause a VM exit. As such, time
>  spent while a virtual CPU is halted in this way will then be accounted for as
> -guest running time on the host (as opposed to e.g. HLT).
> +guest running time on the host.
> +
> +7.14 KVM_CAP_X86_GUEST_HLT
> +
> +Architectures: x86
> +Parameters: none
> +Returns: 0 on success
> +
> +This capability indicates that a guest using HLT to stop a virtual CPU will not
> +cause a VM exit. As such, time spent while a virtual CPU is halted in this way
> +will then be accounted for as guest running time on the host.
>  
>  8. Other capabilities.
>  ----------------------
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index f7bcfaa..3197c2d 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -781,6 +781,7 @@ struct kvm_arch {
>  
>  	gpa_t wall_clock;
>  
> +	bool hlt_in_guest;
>  	bool mwait_in_guest;
>  
>  	bool ept_identity_pagetable_done;
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index ef1b320..c135b98 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -1236,7 +1236,6 @@ static void init_vmcb(struct vcpu_svm *svm)
>  	set_intercept(svm, INTERCEPT_RDPMC);
>  	set_intercept(svm, INTERCEPT_CPUID);
>  	set_intercept(svm, INTERCEPT_INVD);
> -	set_intercept(svm, INTERCEPT_HLT);
>  	set_intercept(svm, INTERCEPT_INVLPG);
>  	set_intercept(svm, INTERCEPT_INVLPGA);
>  	set_intercept(svm, INTERCEPT_IOIO_PROT);
> @@ -1257,6 +1256,8 @@ static void init_vmcb(struct vcpu_svm *svm)
>  		set_intercept(svm, INTERCEPT_MONITOR);
>  		set_intercept(svm, INTERCEPT_MWAIT);
>  	}
> +	if (!kvm_hlt_in_guest(svm->vcpu.kvm))
> +		set_intercept(svm, INTERCEPT_HLT);
>  
>  	control->iopm_base_pa = __sme_set(iopm_base);
>  	control->msrpm_base_pa = __sme_set(__pa(svm->msrpm));
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index a067735..1b67433 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -2446,6 +2446,25 @@ static void skip_emulated_instruction(struct kvm_vcpu *vcpu)
>  	vmx_set_interrupt_shadow(vcpu, 0);
>  }
>  
> +static void vmx_set_intr_info(struct kvm_vcpu *vcpu, u32 intr)
> +{
> +	vmcs_write32(VM_ENTRY_INTR_INFO_FIELD, intr);
> +
> +	/*
> +	 * Ensure that we clear the HLT state in the VMCS.  We don't need to
> +	 * explicitly skip the instruction because if the HLT state is set, then
> +	 * the instruction is already executing and RIP has already been
> +	 * advanced.
> +	 */
> +	if (!kvm_hlt_in_guest(vcpu->kvm) || !(intr & INTR_INFO_VALID_MASK))
> +		return;
> +	if (is_external_interrupt(intr) || is_nmi(intr))
> +		return;
> +	if (vmcs_read32(GUEST_ACTIVITY_STATE) != GUEST_ACTIVITY_HLT)
> +		return;
> +	vmcs_write32(GUEST_ACTIVITY_STATE, GUEST_ACTIVITY_ACTIVE);
> +}
> +
>  static void nested_vmx_inject_exception_vmexit(struct kvm_vcpu *vcpu,
>  					       unsigned long exit_qual)
>  {
> @@ -2540,7 +2559,7 @@ static void vmx_queue_exception(struct kvm_vcpu *vcpu)
>  	} else
>  		intr_info |= INTR_TYPE_HARD_EXCEPTION;
>  
> -	vmcs_write32(VM_ENTRY_INTR_INFO_FIELD, intr_info);
> +	vmx_set_intr_info(vcpu, intr_info);
>  }
>  
>  static bool vmx_rdtscp_supported(void)
> @@ -5298,6 +5317,8 @@ static u32 vmx_exec_control(struct vcpu_vmx *vmx)
>  	if (kvm_mwait_in_guest(vmx->vcpu.kvm))
>  		exec_control &= ~(CPU_BASED_MWAIT_EXITING |
>  				  CPU_BASED_MONITOR_EXITING);
> +	if (kvm_hlt_in_guest(vmx->vcpu.kvm))
> +		exec_control &= ~CPU_BASED_HLT_EXITING;
>  	return exec_control;
>  }
>  
> @@ -5635,7 +5656,7 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>  
>  	setup_msrs(vmx);
>  
> -	vmcs_write32(VM_ENTRY_INTR_INFO_FIELD, 0);  /* 22.2.1 */
> +	vmx_set_intr_info(vcpu, 0);  /* 22.2.1 */
>  
>  	if (cpu_has_vmx_tpr_shadow() && !init_event) {
>  		vmcs_write64(VIRTUAL_APIC_PAGE_ADDR, 0);
> @@ -5729,7 +5750,7 @@ static void vmx_inject_irq(struct kvm_vcpu *vcpu)
>  			     vmx->vcpu.arch.event_exit_inst_len);
>  	} else
>  		intr |= INTR_TYPE_EXT_INTR;
> -	vmcs_write32(VM_ENTRY_INTR_INFO_FIELD, intr);
> +	vmx_set_intr_info(vcpu, intr);
>  }
>  
>  static void vmx_inject_nmi(struct kvm_vcpu *vcpu)
> @@ -5758,8 +5779,8 @@ static void vmx_inject_nmi(struct kvm_vcpu *vcpu)
>  		return;
>  	}
>  
> -	vmcs_write32(VM_ENTRY_INTR_INFO_FIELD,
> -			INTR_TYPE_NMI_INTR | INTR_INFO_VALID_MASK | NMI_VECTOR);
> +	vmx_set_intr_info(vcpu, INTR_TYPE_NMI_INTR | INTR_INFO_VALID_MASK |
> +				NMI_VECTOR);
>  }
>  
>  static bool vmx_get_nmi_mask(struct kvm_vcpu *vcpu)
> @@ -9301,7 +9322,7 @@ static void vmx_cancel_injection(struct kvm_vcpu *vcpu)
>  				  VM_ENTRY_INSTRUCTION_LEN,
>  				  VM_ENTRY_EXCEPTION_ERROR_CODE);
>  
> -	vmcs_write32(VM_ENTRY_INTR_INFO_FIELD, 0);
> +	vmx_set_intr_info(vcpu, 0);
>  }
>  
>  static void atomic_switch_perf_msrs(struct vcpu_vmx *vmx)
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index fe6627a..f17c520 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2755,6 +2755,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>  	case KVM_CAP_SET_BOOT_CPU_ID:
>   	case KVM_CAP_SPLIT_IRQCHIP:
>  	case KVM_CAP_IMMEDIATE_EXIT:
> +	case KVM_CAP_X86_GUEST_HLT:
>  		r = 1;
>  		break;
>  	case KVM_CAP_ADJUST_CLOCK:
> @@ -4068,6 +4069,10 @@ static int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
>  			r = 0;
>  		}
>  		break;
> +	case KVM_CAP_X86_GUEST_HLT:
> +		kvm->arch.hlt_in_guest = true;
> +		r = 0;
> +		break;
>  	default:
>  		r = -EINVAL;
>  		break;
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index ed8e150..b2066aa 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -266,4 +266,9 @@ static inline bool kvm_mwait_in_guest(struct kvm *kvm)
>  	return kvm->arch.mwait_in_guest;
>  }
>  
> +static inline bool kvm_hlt_in_guest(struct kvm *kvm)
> +{
> +	return kvm->arch.hlt_in_guest;
> +}
> +
>  #endif
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 282d7613..ff8f266 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -932,6 +932,7 @@ struct kvm_ppc_resize_hpt {
>  #define KVM_CAP_HYPERV_SYNIC2 148
>  #define KVM_CAP_HYPERV_VP_INDEX 149
>  #define KVM_CAP_S390_AIS_MIGRATION 150
> +#define KVM_CAP_X86_GUEST_HLT 151
>  
>  #ifdef KVM_CAP_IRQ_ROUTING
>  
> -- 
> 2.3.1.dirty