Date:   Wed, 20 Nov 2019 11:49:36 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Liran Alon <liran.alon@...cle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        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>
Subject: Re: [PATCH v2 1/2] KVM: VMX: FIXED+PHYSICAL mode single target IPI fastpath

On Tue, 19 Nov 2019 at 20:11, Liran Alon <liran.alon@...cle.com> wrote:
>
>
>
> > On 19 Nov 2019, at 8:36, 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 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@...cent.com>
> > ---
> > 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
> >
> > #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)
>
> vmx_handle_exit() should get second parameter by value and not by pointer. As it doesn’t need to modify it.
>
> I would also rename parameter to “accel_exit_completion” to indicate this is additional work that needs to happen to complete accelerated-exit handling.
> This parameter should be an enum that currently only have 2 values: ACCEL_EXIT_NONE and ACCEL_EXIT_SKIP_EMUL_INS.
>
> > {
> >       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) &&
> > +                     ((data & KVM_APIC_DEST_MASK) == APIC_DEST_PHYSICAL) &&
> > +                     ((data & APIC_MODE_MASK) == APIC_DM_FIXED)) {
> > +
> > +                     trace_kvm_msr_write(index, data);
>
> On a standard EXIT_REASON_MSR_WRITE VMExit, this trace will be printed only after LAPIC emulation logic happens.
> You should preserve same ordering.
>
> > +                     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;
> > +}
>
> Maybe it would be more elegant to modify this function as follows?
>
> static int handle_accel_set_x2apic_icr_irqoff(struct kvm_vcpu *vcpu, u32 msr, 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;
> }
>
> static 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, msr, 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;
> }
>
> > +
> > +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);
>
> 1) This case requires a comment as the only reason it is called here is an optimisation.
> In contrast to the other cases which must be called before interrupts are enabled on the host.
>
> 2) I would rename handler to handle_accel_set_msr_irqoff().
> To signal this handler runs with host interrupts disabled and to make it a general place for accelerating WRMSRs in case we would require more in the future.

Yes, TSCDEADLINE/VMX PREEMPTION TIMER is in my todo list after this
merged upstream, handle all the comments in v3, thanks for making this
nicer further. :)

    Wanpeng

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