Re: [PATCH v2 1/2] KVM: VMX: FIXED+PHYSICAL mode single target IPI fastpath

[Vitaly Kuznetsov]

Wanpeng Li <kernellwp@xxxxxxxxx> writes:

> From: Wanpeng Li <wanpengli@xxxxxxxxxxx>
>
> ICR and TSCDEADLINE MSRs write cause the main MSRs write vmexits in 
> our product observation, multicast IPIs are not as common as unicast 
> IPI like RESCHEDULE_VECTOR and CALL_FUNCTION_SINGLE_VECTOR etc.
>
> This patch tries to optimize x2apic physical destination mode, fixed 
> delivery mode single target IPI by delivering IPI to receiver as soon 
> as possible after sender writes ICR vmexit to avoid various checks 
> when possible, especially when running guest w/ --overcommit cpu-pm=on
> or guest can keep running, IPI can be injected to target vCPU by 
> posted-interrupt immediately.
>
> Testing on Xeon Skylake server:
>
> The virtual IPI latency from sender send to receiver receive reduces 
> more than 200+ cpu cycles.
>
> Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> ---
> v1 -> v2:
>  * add tracepoint
>  * Instead of a separate vcpu->fast_vmexit, set exit_reason
>    to vmx->exit_reason to -1 if the fast path succeeds.
>  * move the "kvm_skip_emulated_instruction(vcpu)" to vmx_handle_exit
>  * moving the handling into vmx_handle_exit_irqoff()
>
>  arch/x86/include/asm/kvm_host.h |  4 ++--
>  arch/x86/include/uapi/asm/vmx.h |  1 +
>  arch/x86/kvm/svm.c              |  4 ++--
>  arch/x86/kvm/vmx/vmx.c          | 40 +++++++++++++++++++++++++++++++++++++---
>  arch/x86/kvm/x86.c              |  5 +++--
>  5 files changed, 45 insertions(+), 9 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 898ab9e..0daafa9 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1084,7 +1084,7 @@ struct kvm_x86_ops {
>  	void (*tlb_flush_gva)(struct kvm_vcpu *vcpu, gva_t addr);
>  
>  	void (*run)(struct kvm_vcpu *vcpu);
> -	int (*handle_exit)(struct kvm_vcpu *vcpu);
> +	int (*handle_exit)(struct kvm_vcpu *vcpu, u32 *vcpu_exit_reason);
>  	int (*skip_emulated_instruction)(struct kvm_vcpu *vcpu);
>  	void (*set_interrupt_shadow)(struct kvm_vcpu *vcpu, int mask);
>  	u32 (*get_interrupt_shadow)(struct kvm_vcpu *vcpu);
> @@ -1134,7 +1134,7 @@ struct kvm_x86_ops {
>  	int (*check_intercept)(struct kvm_vcpu *vcpu,
>  			       struct x86_instruction_info *info,
>  			       enum x86_intercept_stage stage);
> -	void (*handle_exit_irqoff)(struct kvm_vcpu *vcpu);
> +	void (*handle_exit_irqoff)(struct kvm_vcpu *vcpu, u32 *vcpu_exit_reason);
>  	bool (*mpx_supported)(void);
>  	bool (*xsaves_supported)(void);
>  	bool (*umip_emulated)(void);
> diff --git a/arch/x86/include/uapi/asm/vmx.h b/arch/x86/include/uapi/asm/vmx.h
> index 3eb8411..b33c6e1 100644
> --- a/arch/x86/include/uapi/asm/vmx.h
> +++ b/arch/x86/include/uapi/asm/vmx.h
> @@ -88,6 +88,7 @@
>  #define EXIT_REASON_XRSTORS             64
>  #define EXIT_REASON_UMWAIT              67
>  #define EXIT_REASON_TPAUSE              68
> +#define EXIT_REASON_NEED_SKIP_EMULATED_INSN -1

Maybe just EXIT_REASON_INSN_SKIP ?

>  
>  #define VMX_EXIT_REASONS \
>  	{ EXIT_REASON_EXCEPTION_NMI,         "EXCEPTION_NMI" }, \
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index d02a73a..c8e063a 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -4929,7 +4929,7 @@ static void svm_get_exit_info(struct kvm_vcpu *vcpu, u64 *info1, u64 *info2)
>  	*info2 = control->exit_info_2;
>  }
>  
> -static int handle_exit(struct kvm_vcpu *vcpu)
> +static int handle_exit(struct kvm_vcpu *vcpu, u32 *vcpu_exit_reason)
>  {
>  	struct vcpu_svm *svm = to_svm(vcpu);
>  	struct kvm_run *kvm_run = vcpu->run;
> @@ -6187,7 +6187,7 @@ static int svm_check_intercept(struct kvm_vcpu *vcpu,
>  	return ret;
>  }
>  
> -static void svm_handle_exit_irqoff(struct kvm_vcpu *vcpu)
> +static void svm_handle_exit_irqoff(struct kvm_vcpu *vcpu, u32 *vcpu_exit_reason)
>  {
>  
>  }
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 621142e5..b98198d 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -5792,7 +5792,7 @@ void dump_vmcs(void)
>   * The guest has exited.  See if we can fix it or if we need userspace
>   * assistance.
>   */
> -static int vmx_handle_exit(struct kvm_vcpu *vcpu)
> +static int vmx_handle_exit(struct kvm_vcpu *vcpu, u32 *vcpu_exit_reason)
>  {
>  	struct vcpu_vmx *vmx = to_vmx(vcpu);
>  	u32 exit_reason = vmx->exit_reason;
> @@ -5878,7 +5878,10 @@ static int vmx_handle_exit(struct kvm_vcpu *vcpu)
>  		}
>  	}
>  
> -	if (exit_reason < kvm_vmx_max_exit_handlers
> +	if (*vcpu_exit_reason == EXIT_REASON_NEED_SKIP_EMULATED_INSN) {
> +		kvm_skip_emulated_instruction(vcpu);
> +		return 1;
> +	} else if (exit_reason < kvm_vmx_max_exit_handlers
>  	    && kvm_vmx_exit_handlers[exit_reason]) {
>  #ifdef CONFIG_RETPOLINE
>  		if (exit_reason == EXIT_REASON_MSR_WRITE)
> @@ -6223,7 +6226,36 @@ static void handle_external_interrupt_irqoff(struct kvm_vcpu *vcpu)
>  }
>  STACK_FRAME_NON_STANDARD(handle_external_interrupt_irqoff);
>  
> -static void vmx_handle_exit_irqoff(struct kvm_vcpu *vcpu)
> +static u32 handle_ipi_fastpath(struct kvm_vcpu *vcpu)
> +{
> +	u32 index;
> +	u64 data;
> +	int ret = 0;
> +
> +	if (lapic_in_kernel(vcpu) && apic_x2apic_mode(vcpu->arch.apic)) {
> +		/*
> +		 * fastpath to IPI target, FIXED+PHYSICAL which is popular
> +		 */
> +		index = kvm_rcx_read(vcpu);
> +		data = kvm_read_edx_eax(vcpu);
> +
> +		if (((index - APIC_BASE_MSR) << 4 == APIC_ICR) &&

What if index (RCX) is < APIC_BASE_MSR?

> +			((data & KVM_APIC_DEST_MASK) == APIC_DEST_PHYSICAL) &&
> +			((data & APIC_MODE_MASK) == APIC_DM_FIXED)) {
> +
> +			trace_kvm_msr_write(index, data);
> +			kvm_lapic_set_reg(vcpu->arch.apic, APIC_ICR2, (u32)(data >> 32));
> +			ret = kvm_lapic_reg_write(vcpu->arch.apic, APIC_ICR, (u32)data);
> +
> +			if (ret == 0)
> +				return EXIT_REASON_NEED_SKIP_EMULATED_INSN;
> +		}
> +	}
> +
> +	return ret;
> +}
> +
> +static void vmx_handle_exit_irqoff(struct kvm_vcpu *vcpu, u32 *exit_reason)
>  {
>  	struct vcpu_vmx *vmx = to_vmx(vcpu);
>  
> @@ -6231,6 +6263,8 @@ static void vmx_handle_exit_irqoff(struct kvm_vcpu *vcpu)
>  		handle_external_interrupt_irqoff(vcpu);
>  	else if (vmx->exit_reason == EXIT_REASON_EXCEPTION_NMI)
>  		handle_exception_nmi_irqoff(vmx);
> +	else if (vmx->exit_reason == EXIT_REASON_MSR_WRITE)
> +		*exit_reason = handle_ipi_fastpath(vcpu);
>  }
>  
>  static bool vmx_has_emulated_msr(int index)
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 991dd01..a53bce3 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -7981,6 +7981,7 @@ EXPORT_SYMBOL_GPL(__kvm_request_immediate_exit);
>  static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  {
>  	int r;
> +	u32 exit_reason = 0;
>  	bool req_int_win =
>  		dm_request_for_irq_injection(vcpu) &&
>  		kvm_cpu_accept_dm_intr(vcpu);
> @@ -8226,7 +8227,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  	vcpu->mode = OUTSIDE_GUEST_MODE;
>  	smp_wmb();
>  
> -	kvm_x86_ops->handle_exit_irqoff(vcpu);
> +	kvm_x86_ops->handle_exit_irqoff(vcpu, &exit_reason);
>  
>  	/*
>  	 * Consume any pending interrupts, including the possible source of
> @@ -8270,7 +8271,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  		kvm_lapic_sync_from_vapic(vcpu);
>  
>  	vcpu->arch.gpa_available = false;
> -	r = kvm_x86_ops->handle_exit(vcpu);
> +	r = kvm_x86_ops->handle_exit(vcpu, &exit_reason);
>  	return r;
>  
>  cancel_injection:

-- 
Vitaly