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Date: Thu, 28 Nov 2019 08:27:37 +0800
From: Wanpeng Li <kernellwp@...il.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
Radim Krčmář <rkrcmar@...hat.com>,
Sean Christopherson <sean.j.christopherson@...el.com>,
Vitaly Kuznetsov <vkuznets@...hat.com>,
Wanpeng Li <wanpengli@...cent.com>,
Jim Mattson <jmattson@...gle.com>,
Joerg Roedel <joro@...tes.org>,
Liran Alon <liran.alon@...cle.com>
Subject: Re: [PATCH v4 1/2] KVM: VMX: FIXED+PHYSICAL mode single target IPI fastpath
ping to catch the second week of the merge window. :)
On Thu, 21 Nov 2019 at 11:17, Wanpeng Li <kernellwp@...il.com> wrote:
>
> From: Wanpeng Li <wanpengli@...cent.com>
>
> 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 introduce a mechanism to handle certain performance-critical
> WRMSRs in a very early stage of KVM VMExit handler.
>
> This mechanism is specifically used for accelerating writes to x2APIC ICR
> that attempt to send a virtual IPI with physical destination-mode, fixed
> delivery-mode and single target. Which was found as one of the main causes
> of VMExits for Linux workloads.
>
> The reason this mechanism significantly reduce the latency of such virtual
> IPIs is by sending the physical IPI to the target vCPU in a very early stage
> of KVM VMExit handler, before host interrupts are enabled and before expensive
> operations such as reacquiring KVM’s SRCU lock.
> Latency is reduced even more when KVM is able to use APICv posted-interrupt
> mechanism (which allows to deliver the virtual IPI directly to target vCPU
> without the need to kick it to host).
>
> Testing on Xeon Skylake server:
>
> The virtual IPI latency from sender send to receiver receive reduces
> more than 200+ cpu cycles.
>
> Reviewed-by: Liran Alon <liran.alon@...cle.com>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Cc: Sean Christopherson <sean.j.christopherson@...el.com>
> Cc: Vitaly Kuznetsov <vkuznets@...hat.com>
> Cc: Liran Alon <liran.alon@...cle.com>
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
> v3 -> v4:
> * check !is_guest_mode(vcpu)
> * ACCEL_EXIT_SKIP_EMUL_INS don't need be -1
> * move comments on top of handle_accel_set_x2apic_icr_irqoff
> * update patch description
> v2 -> v3:
> * for both VMX and SVM
> * vmx_handle_exit() get second parameter by value and not by pointer
> * rename parameter to “accel_exit_completion”
> * preserve tracepoint ordering
> * rename handler to handle_accel_set_msr_irqoff and more generic
> * add comments above handle_accel_set_msr_irqoff
> * msr index APIC_BASE_MSR + (APIC_ICR >> 4)
> 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 | 11 ++++++++--
> arch/x86/kvm/svm.c | 15 +++++++++----
> arch/x86/kvm/vmx/vmx.c | 14 +++++++++---
> arch/x86/kvm/x86.c | 48 +++++++++++++++++++++++++++++++++++++++--
> arch/x86/kvm/x86.h | 1 +
> 5 files changed, 78 insertions(+), 11 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 898ab9e..62af1c5 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -175,6 +175,11 @@ enum {
> VCPU_SREG_LDTR,
> };
>
> +enum accel_exit_completion {
> + ACCEL_EXIT_NONE,
> + ACCEL_EXIT_SKIP_EMUL_INS,
> +};
> +
> #include <asm/kvm_emulate.h>
>
> #define KVM_NR_MEM_OBJS 40
> @@ -1084,7 +1089,8 @@ 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,
> + enum accel_exit_completion accel_exit);
> 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 +1140,8 @@ 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,
> + enum accel_exit_completion *accel_exit);
> bool (*mpx_supported)(void);
> bool (*xsaves_supported)(void);
> bool (*umip_emulated)(void);
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index d02a73a..d0367c4 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -4929,7 +4929,8 @@ 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,
> + enum accel_exit_completion accel_exit)
> {
> struct vcpu_svm *svm = to_svm(vcpu);
> struct kvm_run *kvm_run = vcpu->run;
> @@ -4987,7 +4988,10 @@ static int handle_exit(struct kvm_vcpu *vcpu)
> __func__, svm->vmcb->control.exit_int_info,
> exit_code);
>
> - if (exit_code >= ARRAY_SIZE(svm_exit_handlers)
> + if (accel_exit == ACCEL_EXIT_SKIP_EMUL_INS) {
> + kvm_skip_emulated_instruction(vcpu);
> + return 1;
> + } else if (exit_code >= ARRAY_SIZE(svm_exit_handlers)
> || !svm_exit_handlers[exit_code]) {
> vcpu_unimpl(vcpu, "svm: unexpected exit reason 0x%x\n", exit_code);
> dump_vmcb(vcpu);
> @@ -6187,9 +6191,12 @@ 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,
> + enum accel_exit_completion *accel_exit)
> {
> -
> + if (!is_guest_mode(vcpu) &&
> + to_svm(vcpu)->vmcb->control.exit_code == EXIT_REASON_MSR_WRITE)
> + *accel_exit = handle_accel_set_msr_irqoff(vcpu);
> }
>
> static void svm_sched_in(struct kvm_vcpu *vcpu, int cpu)
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 621142e5..5d77188 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -5792,7 +5792,8 @@ 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,
> + enum accel_exit_completion accel_exit)
> {
> struct vcpu_vmx *vmx = to_vmx(vcpu);
> u32 exit_reason = vmx->exit_reason;
> @@ -5878,7 +5879,10 @@ static int vmx_handle_exit(struct kvm_vcpu *vcpu)
> }
> }
>
> - if (exit_reason < kvm_vmx_max_exit_handlers
> + if (accel_exit == ACCEL_EXIT_SKIP_EMUL_INS) {
> + 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 +6227,8 @@ 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 void vmx_handle_exit_irqoff(struct kvm_vcpu *vcpu,
> + enum accel_exit_completion *accel_exit)
> {
> struct vcpu_vmx *vmx = to_vmx(vcpu);
>
> @@ -6231,6 +6236,9 @@ 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 (!is_guest_mode(vcpu) &&
> + vmx->exit_reason == EXIT_REASON_MSR_WRITE)
> + *accel_exit = handle_accel_set_msr_irqoff(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..c55348c 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1510,6 +1510,49 @@ int kvm_emulate_wrmsr(struct kvm_vcpu *vcpu)
> EXPORT_SYMBOL_GPL(kvm_emulate_wrmsr);
>
> /*
> + * The fast path for frequent and performance sensitive wrmsr emulation,
> + * i.e. the sending of IPI, sending IPI early in the VM-Exit flow reduces
> + * the latency of virtual IPI by avoiding the expensive bits of transitioning
> + * from guest to host, e.g. reacquiring KVM's SRCU lock. In contrast to the
> + * other cases which must be called after interrupts are enabled on the host.
> + */
> +static int handle_accel_set_x2apic_icr_irqoff(struct kvm_vcpu *vcpu, u64 data)
> +{
> + if (lapic_in_kernel(vcpu) && apic_x2apic_mode(vcpu->arch.apic) &&
> + ((data & KVM_APIC_DEST_MASK) == APIC_DEST_PHYSICAL) &&
> + ((data & APIC_MODE_MASK) == APIC_DM_FIXED)) {
> +
> + kvm_lapic_set_reg(vcpu->arch.apic, APIC_ICR2, (u32)(data >> 32));
> + return kvm_lapic_reg_write(vcpu->arch.apic, APIC_ICR, (u32)data);
> + }
> +
> + return 1;
> +}
> +
> +enum accel_exit_completion handle_accel_set_msr_irqoff(struct kvm_vcpu *vcpu)
> +{
> + u32 msr = kvm_rcx_read(vcpu);
> + u64 data = kvm_read_edx_eax(vcpu);
> + int ret = 0;
> +
> + switch (msr) {
> + case APIC_BASE_MSR + (APIC_ICR >> 4):
> + ret = handle_accel_set_x2apic_icr_irqoff(vcpu, data);
> + break;
> + default:
> + return ACCEL_EXIT_NONE;
> + }
> +
> + if (!ret) {
> + trace_kvm_msr_write(msr, data);
> + return ACCEL_EXIT_SKIP_EMUL_INS;
> + }
> +
> + return ACCEL_EXIT_NONE;
> +}
> +EXPORT_SYMBOL_GPL(handle_accel_set_msr_irqoff);
> +
> +/*
> * Adapt set_msr() to msr_io()'s calling convention
> */
> static int do_get_msr(struct kvm_vcpu *vcpu, unsigned index, u64 *data)
> @@ -7984,6 +8027,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> bool req_int_win =
> dm_request_for_irq_injection(vcpu) &&
> kvm_cpu_accept_dm_intr(vcpu);
> + enum accel_exit_completion accel_exit = ACCEL_EXIT_NONE;
>
> bool req_immediate_exit = false;
>
> @@ -8226,7 +8270,7 @@ 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, &accel_exit);
>
> /*
> * Consume any pending interrupts, including the possible source of
> @@ -8270,7 +8314,7 @@ 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, accel_exit);
> return r;
>
> cancel_injection:
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index 29391af..f14ec14 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -291,6 +291,7 @@ bool kvm_mtrr_check_gfn_range_consistency(struct kvm_vcpu *vcpu, gfn_t gfn,
> bool kvm_vector_hashing_enabled(void);
> int x86_emulate_instruction(struct kvm_vcpu *vcpu, unsigned long cr2,
> int emulation_type, void *insn, int insn_len);
> +enum accel_exit_completion handle_accel_set_msr_irqoff(struct kvm_vcpu *vcpu);
>
> #define KVM_SUPPORTED_XCR0 (XFEATURE_MASK_FP | XFEATURE_MASK_SSE \
> | XFEATURE_MASK_YMM | XFEATURE_MASK_BNDREGS \
> --
> 2.7.4
>
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