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Message-ID: <CANRm+Cz6NE_72V-_sCQxEvHQxuHEtBVsmYX1tE02+Y6EPRFzkA@mail.gmail.com>
Date: Sat, 8 Jan 2022 10:08:59 +0800
From: Wanpeng Li <kernellwp@...il.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
Paolo Bonzini <pbonzini@...hat.com>,
Vitaly Kuznetsov <vkuznets@...hat.com>,
Wanpeng Li <wanpengli@...cent.com>,
Jim Mattson <jmattson@...gle.com>,
Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH] KVM: VMX: Dont' deliver posted IRQ if vCPU == this vCPU
and vCPU is IN_GUEST_MODE
On Sat, 8 Jan 2022 at 08:17, Sean Christopherson <seanjc@...gle.com> wrote:
>
> Nit, s/deliver/send, "deliver" reads as though KVM is ignoring an event that was
> sent by something else.
>
> On Thu, Jan 06, 2022, Wanpeng Li wrote:
> > From: Wanpeng Li <wanpengli@...cent.com>
> >
> > Commit fdba608f15e2 (KVM: VMX: Wake vCPU when delivering posted IRQ even
> > if vCPU == this vCPU) fixes wakeup event is missing when it is not from
> > synchronous kvm context by dropping vcpu == running_vcpu checking completely.
> > However, it will break the original goal to optimise timer fastpath, let's
> > move the checking under vCPU is IN_GUEST_MODE to restore the performance.
>
> Please (a) explain why this is safe and (b) provide context for exactly what
> fastpath this helpers. Lack of context is partly what led to the optimization
> being reverted instead of being fixed as below, and forcing readers to jump through
> multiple changelogs to understand what's going on is unnecessarily mean.
>
> E.g.
>
> 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").
>
> The comment above send_IPI_mask() also needs to be updated. There are a few
> existing grammar and style nits that can be opportunistically cleaned up, too.
>
> Paolo, if Wanpeng doesn't object, can you use the above changelog and the below
> comment?
Thanks for these updates, Sean.
Wanpeng
>
> With that,
>
> Reviewed-by: Sean Christopherson <seanjc@...gle.com>
>
> ---
> arch/x86/kvm/vmx/vmx.c | 41 +++++++++++++++++++++--------------------
> 1 file changed, 21 insertions(+), 20 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index fe06b02994e6..730df0e183d6 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -3908,31 +3908,32 @@ 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
>
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