Re: [PATCH v2] KVM: X86: Ultra fast single target IPI fastpath
From: Sean Christopherson
Date: Fri Apr 10 2020 - 11:35:42 EST
On Fri, Apr 10, 2020 at 09:03:27AM +0800, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@xxxxxxxxxxx>
>
> IPI and Timer cause the main MSRs write vmexits in cloud environment
> observation, let's optimize virtual IPI latency more aggressively to
> inject target IPI as soon as possible.
>
> Running kvm-unit-tests/vmexit.flat IPI testing on SKX server, disable
> adaptive advance lapic timer and adaptive halt-polling to avoid the
> interference, this patch can give another 7% improvement.
>
> w/o fastpath -> fastpath 4238 -> 3543 16.4%
> fastpath -> ultra fastpath 3543 -> 3293 7%
> w/o fastpath -> ultra fastpath 4238 -> 3293 22.3%
>
> This also revises the performance data in commit 1e9e2622a1 (KVM: VMX:
> FIXED+PHYSICAL mode single target IPI fastpath), that testing adds
> --overcommit cpu-pm=on to kvm-unit-tests guest which is unnecessary.
>
> Tested-by: Haiwei Li <lihaiwei@xxxxxxxxxxx>
> Cc: Haiwei Li <lihaiwei@xxxxxxxxxxx>
> Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> ---
> v1 -> v2:
> * rebase on latest kvm/queue
> * update patch description
>
> arch/x86/include/asm/kvm_host.h | 6 +++---
> arch/x86/kvm/svm/svm.c | 21 ++++++++++++++-------
> arch/x86/kvm/vmx/vmx.c | 19 +++++++++++++------
> arch/x86/kvm/x86.c | 4 ++--
> 4 files changed, 32 insertions(+), 18 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index c7da23a..e667cf3 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1124,7 +1124,8 @@ struct kvm_x86_ops {
> */
> void (*tlb_flush_guest)(struct kvm_vcpu *vcpu);
>
> - void (*run)(struct kvm_vcpu *vcpu);
> + void (*run)(struct kvm_vcpu *vcpu,
> + enum exit_fastpath_completion *exit_fastpath);
> int (*handle_exit)(struct kvm_vcpu *vcpu,
> enum exit_fastpath_completion exit_fastpath);
> int (*skip_emulated_instruction)(struct kvm_vcpu *vcpu);
> @@ -1174,8 +1175,7 @@ struct kvm_x86_ops {
> struct x86_instruction_info *info,
> enum x86_intercept_stage stage,
> struct x86_exception *exception);
> - void (*handle_exit_irqoff)(struct kvm_vcpu *vcpu,
> - enum exit_fastpath_completion *exit_fastpath);
> + void (*handle_exit_irqoff)(struct kvm_vcpu *vcpu);
>
> int (*check_nested_events)(struct kvm_vcpu *vcpu);
> void (*request_immediate_exit)(struct kvm_vcpu *vcpu);
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 27f4684..c019332 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -3283,9 +3283,20 @@ static void svm_cancel_injection(struct kvm_vcpu *vcpu)
> svm_complete_interrupts(svm);
> }
>
> +static enum exit_fastpath_completion svm_exit_handlers_fastpath(struct kvm_vcpu *vcpu)
> +{
> + if (!is_guest_mode(vcpu) &&
> + to_svm(vcpu)->vmcb->control.exit_code == SVM_EXIT_MSR &&
> + to_svm(vcpu)->vmcb->control.exit_info_1)
> + return handle_fastpath_set_msr_irqoff(vcpu);
> +
> + return EXIT_FASTPATH_NONE;
> +}
> +
> bool __svm_vcpu_run(unsigned long vmcb_pa, unsigned long *regs);
>
> -static void svm_vcpu_run(struct kvm_vcpu *vcpu)
> +static void svm_vcpu_run(struct kvm_vcpu *vcpu,
> + enum exit_fastpath_completion *exit_fastpath)
> {
> struct vcpu_svm *svm = to_svm(vcpu);
>
> @@ -3388,6 +3399,7 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
> kvm_load_host_xsave_state(vcpu);
> stgi();
>
> + *exit_fastpath = svm_exit_handlers_fastpath(vcpu);
> /* Any pending NMI will happen here */
>
> if (unlikely(svm->vmcb->control.exit_code == SVM_EXIT_NMI))
> @@ -3719,13 +3731,8 @@ static int svm_check_intercept(struct kvm_vcpu *vcpu,
> return ret;
> }
>
> -static void svm_handle_exit_irqoff(struct kvm_vcpu *vcpu,
> - enum exit_fastpath_completion *exit_fastpath)
> +static void svm_handle_exit_irqoff(struct kvm_vcpu *vcpu)
> {
> - if (!is_guest_mode(vcpu) &&
> - to_svm(vcpu)->vmcb->control.exit_code == SVM_EXIT_MSR &&
> - to_svm(vcpu)->vmcb->control.exit_info_1)
> - *exit_fastpath = handle_fastpath_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 1d2bb57..61a1725 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6354,8 +6354,7 @@ 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,
> - enum exit_fastpath_completion *exit_fastpath)
> +static void vmx_handle_exit_irqoff(struct kvm_vcpu *vcpu)
> {
> struct vcpu_vmx *vmx = to_vmx(vcpu);
>
> @@ -6363,9 +6362,6 @@ 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)
> - *exit_fastpath = handle_fastpath_set_msr_irqoff(vcpu);
> }
>
> static bool vmx_has_emulated_msr(int index)
> @@ -6570,9 +6566,19 @@ void vmx_update_host_rsp(struct vcpu_vmx *vmx, unsigned long host_rsp)
> }
> }
>
> +static enum exit_fastpath_completion vmx_exit_handlers_fastpath(struct kvm_vcpu *vcpu)
> +{
> + if (!is_guest_mode(vcpu) &&
> + to_vmx(vcpu)->exit_reason == EXIT_REASON_MSR_WRITE)
Bad indentation.
> + return handle_fastpath_set_msr_irqoff(vcpu);
> +
> + return EXIT_FASTPATH_NONE;
> +}
> +
> bool __vmx_vcpu_run(struct vcpu_vmx *vmx, unsigned long *regs, bool launched);
>
> -static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
> +static void vmx_vcpu_run(struct kvm_vcpu *vcpu,
> + enum exit_fastpath_completion *exit_fastpath)
Why pass a pointer instead of returning the enum?
> {
> struct vcpu_vmx *vmx = to_vmx(vcpu);
> unsigned long cr3, cr4;
> @@ -6737,6 +6743,7 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
> vmx->idt_vectoring_info = 0;
>
> vmx->exit_reason = vmx->fail ? 0xdead : vmcs_read32(VM_EXIT_REASON);
> + *exit_fastpath = vmx_exit_handlers_fastpath(vcpu);
IMO, this should come at the very end of vmx_vcpu_run(). At a minimum, it
needs to be moved below the #MC handling and below
if (vmx->fail || (vmx->exit_reason & VMX_EXIT_REASONS_FAILED_VMENTRY))
return;
KVM more or less assumes vmx->idt_vectoring_info is always valid, and it's
not obvious that a generic fastpath call can safely run before
vmx_complete_interrupts(), e.g. the kvm_clear_interrupt_queue() call.
In a normal scenario, the added latency is <50 cycles. ~30 for the VMREAD
of IDT_VECTORING_INFO_FIELD, a handful of zeroing instructions, and a few
CMP+Jcc style uops to skip NMI blocking and interrupt completion.
And if the result is returned, it means VMX won't need a local variable, e.g.:
vmx->exit_reason = vmx->fail ? 0xdead : vmcs_read32(VM_EXIT_REASON);
if ((u16)vmx->exit_reason == EXIT_REASON_MCE_DURING_VMENTRY)
kvm_machine_check();
if (vmx->fail || (vmx->exit_reason & VMX_EXIT_REASONS_FAILED_VMENTRY))
return EXIT_FASTPATH_NONE;
vmx->loaded_vmcs->launched = 1;
vmx->idt_vectoring_info = vmcs_read32(IDT_VECTORING_INFO_FIELD);
vmx_recover_nmi_blocking(vmx);
vmx_complete_interrupts(vmx);
return vmx_exit_handlers_fastpath(vcpu);
}
> if ((u16)vmx->exit_reason == EXIT_REASON_MCE_DURING_VMENTRY)
> kvm_machine_check();
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 3089aa4..eed31e2 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8409,7 +8409,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_RELOAD;
> }
>
> - kvm_x86_ops.run(vcpu);
> + kvm_x86_ops.run(vcpu, &exit_fastpath);
Pretty sre
>
> /*
> * Do this here before restoring debug registers on the host. And
> @@ -8441,7 +8441,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> vcpu->mode = OUTSIDE_GUEST_MODE;
> smp_wmb();
>
> - kvm_x86_ops.handle_exit_irqoff(vcpu, &exit_fastpath);
> + kvm_x86_ops.handle_exit_irqoff(vcpu);
>
> /*
> * Consume any pending interrupts, including the possible source of
> --
> 2.7.4
>