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

From: Wanpeng Li
Date: Tue Nov 19 2019 - 06:59:43 EST

On Tue, 19 Nov 2019 at 19:54, Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> wrote:
>
> 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?

How about if (index == (APIC_BASE_MSR + 0x300) &&

>
> > + ((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
>