linux-kernel - Re: [PATCH v2] KVM: VMX: Dont' send posted IRQ if vCPU == this vCPU and vCPU is IN_GUEST

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Date:   Wed, 19 Jan 2022 15:11:44 +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>,
        Sean Christopherson <seanjc@...gle.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] KVM: VMX: Dont' send posted IRQ if vCPU == this vCPU
 and vCPU is IN_GUEST_MODE

kindly ping, :)
On Sat, 8 Jan 2022 at 10:43, Wanpeng Li <kernellwp@...il.com> wrote:
>
> From: Wanpeng Li <wanpengli@...cent.com>
>
> When delivering a virtual interrupt, don't actually send a posted interrupt
> if the target vCPU is also the currently running vCPU and is IN_GUEST_MODE,
> in which case the interrupt is being sent from a VM-Exit fastpath and the
> core run loop in vcpu_enter_guest() will manually move the interrupt from
> the PIR to vmcs.GUEST_RVI.  IRQs are disabled while IN_GUEST_MODE, thus
> there's no possibility of the virtual interrupt being sent from anything
> other than KVM, i.e. KVM won't suppress a wake event from an IRQ handler
> (see commit fdba608f15e2, "KVM: VMX: Wake vCPU when delivering posted IRQ
> even if vCPU == this vCPU").
>
> Eliding the posted interrupt restores the performance provided by the
> combination of commits 379a3c8ee444 ("KVM: VMX: Optimize posted-interrupt
> delivery for timer fastpath") and 26efe2fd92e5 ("KVM: VMX: Handle
> preemption timer fastpath").
>
> Thanks Sean for better comments.
>
> Suggested-by: Chao Gao <chao.gao@...el.com>
> Reviewed-by: Sean Christopherson <seanjc@...gle.com>
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 40 +++++++++++++++++++++-------------------
>  1 file changed, 21 insertions(+), 19 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index fe06b02994e6..e06377c9a4cf 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -3908,31 +3908,33 @@ static inline void kvm_vcpu_trigger_posted_interrupt(struct kvm_vcpu *vcpu,
>  #ifdef CONFIG_SMP
>         if (vcpu->mode == IN_GUEST_MODE) {
>                 /*
> -                * The vector of interrupt to be delivered to vcpu had
> -                * been set in PIR before this function.
> +                * The vector of the virtual has already been set in the PIR.
> +                * Send a notification event to deliver the virtual interrupt
> +                * unless the vCPU is the currently running vCPU, i.e. the
> +                * event is being sent from a fastpath VM-Exit handler, in
> +                * which case the PIR will be synced to the vIRR before
> +                * re-entering the guest.
>                  *
> -                * Following cases will be reached in this block, and
> -                * we always send a notification event in all cases as
> -                * explained below.
> +                * When the target is not the running vCPU, the following
> +                * possibilities emerge:
>                  *
> -                * Case 1: vcpu keeps in non-root mode. Sending a
> -                * notification event posts the interrupt to vcpu.
> +                * Case 1: vCPU stays in non-root mode. Sending a notification
> +                * event posts the interrupt to the vCPU.
>                  *
> -                * Case 2: vcpu exits to root mode and is still
> -                * runnable. PIR will be synced to vIRR before the
> -                * next vcpu entry. Sending a notification event in
> -                * this case has no effect, as vcpu is not in root
> -                * mode.
> +                * Case 2: vCPU exits to root mode and is still runnable. The
> +                * PIR will be synced to the vIRR before re-entering the guest.
> +                * Sending a notification event is ok as the host IRQ handler
> +                * will ignore the spurious event.
>                  *
> -                * Case 3: vcpu exits to root mode and is blocked.
> -                * vcpu_block() has already synced PIR to vIRR and
> -                * never blocks vcpu if vIRR is not cleared. Therefore,
> -                * a blocked vcpu here does not wait for any requested
> -                * interrupts in PIR, and sending a notification event
> -                * which has no effect is safe here.
> +                * Case 3: vCPU exits to root mode and is blocked. vcpu_block()
> +                * has already synced PIR to vIRR and never blocks the vCPU if
> +                * the vIRR is not empty. Therefore, a blocked vCPU here does
> +                * not wait for any requested interrupts in PIR, and sending a
> +                * notification event also results in a benign, spurious event.
>                  */
>
> -               apic->send_IPI_mask(get_cpu_mask(vcpu->cpu), pi_vec);
> +               if (vcpu != kvm_get_running_vcpu())
> +                       apic->send_IPI_mask(get_cpu_mask(vcpu->cpu), pi_vec);
>                 return;
>         }
>  #endif
> --
> 2.25.1
>